Working from diff_output_edscript add support for classic diff output.
The main differences between these two formats is the header
construction for a hunk and selection around which lines that are
output.

Classic diffs denote removed and added lines, and also changes in the
form:

< removed
---
> added

whereas ed scripts only have to included added and changed lines.

---
 lib/diff_output_plain.c | 193 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 174 insertions(+), 19 deletions(-)

diff --git a/lib/diff_output_plain.c b/lib/diff_output_plain.c
index 9053d5e..1ead912 100644
--- a/lib/diff_output_plain.c
+++ b/lib/diff_output_plain.c
@@ -27,42 +27,197 @@
 
 #include "diff_internal.h"
 
+static int
+output_plain_chunk(struct diff_output_info *outinfo,
+    FILE *dest, const struct diff_input_info *info,
+    const struct diff_result *result,
+    struct diff_chunk_context *cc)
+{
+	off_t outoff = 0, *offp;
+	int left_start, left_len, right_start, right_len;
+	int rc;
+	bool change = false;
+
+	left_len = cc->left.end - cc->left.start;
+	if (left_len < 0)
+		return EINVAL;
+	else if (result->left->atoms.len == 0)
+		left_start = 0;
+	else if (left_len == 0 && cc->left.start > 0)
+		left_start = cc->left.start;
+	else if (cc->left.end > 0)
+		left_start = cc->left.start + 1;
+	else
+		left_start = cc->left.start;
+
+	right_len = cc->right.end - cc->right.start;
+	if (right_len < 0)
+		return EINVAL;
+	else if (result->right->atoms.len == 0)
+		right_start = 0;
+	else if (right_len == 0 && cc->right.start > 0)
+		right_start = cc->right.start;
+	else if (cc->right.end > 0)
+		right_start = cc->right.start + 1;
+	else
+		right_start = cc->right.start;
+
+	if (left_len == 0) {
+		/* addition */
+		if (right_len == 1) {
+			rc = fprintf(dest, "%da%d\n", left_start, right_start);
+		} else {
+			rc = fprintf(dest, "%da%d,%d\n", left_start,
+			    right_start, cc->right.end);
+		}
+	} else if (right_len == 0) {
+		/* deletion */
+		if (left_len == 1) {
+			rc = fprintf(dest, "%dd%d\n", left_start,
+			    right_start);
+		} else {
+			rc = fprintf(dest, "%d,%dd%d\n", left_start,
+			    cc->left.end, right_start);
+		}
+	} else {
+		/* change */
+		change = true;
+		if (left_len == 1 && right_len == 1) {
+			rc = fprintf(dest, "%dc%d\n", left_start, right_start);
+		} else if (left_len == 1) {
+			rc = fprintf(dest, "%dc%d,%d\n", left_start,
+			    right_start, cc->right.end);
+		} else if (right_len == 1) {
+			rc = fprintf(dest, "%d,%dc%d\n", left_start,
+			    cc->left.end, right_start);
+		} else {
+			rc = fprintf(dest, "%d,%dc%d,%d\n", left_start,
+			    cc->left.end, right_start, cc->right.end);
+		}
+	}
+
+	/*
+	 * Now write out all the joined chunks.
+	 *
+	 * If the hunk denotes a change, it will come in the form of a deletion
+	 * chunk followed by a addition chunk. Print a marker to break up the
+	 * additions and deletions when this happens.
+	 */
+	int c_idx;
+	for (c_idx = cc->chunk.start; c_idx < cc->chunk.end; c_idx++) {
+		const struct diff_chunk *c = &result->chunks.head[c_idx];
+		if (c->left_count && !c->right_count)
+			rc = diff_output_lines(outinfo, dest,
+					  c->solved ? "< " : "?",
+					  c->left_start, c->left_count);
+		else if (c->right_count && !c->left_count) {
+			if (change)
+				fprintf(dest, "---\n");
+			rc = diff_output_lines(outinfo, dest,
+					  c->solved ? "> " : "?",
+					  c->right_start, c->right_count);
+		}
+		if (rc)
+			return rc;
+		if (cc->chunk.end == result->chunks.len) {
+			rc = diff_output_trailing_newline_msg(outinfo, dest, c);
+			if (rc != DIFF_RC_OK)
+				return rc;
+		}
+	}
+
+	if (rc < 0)
+		return errno;
+	if (outinfo) {
+		ARRAYLIST_ADD(offp, outinfo->line_offsets);
+		if (offp == NULL)
+			return ENOMEM;
+		outoff += rc;
+		*offp = outoff;
+	}
+
+	return DIFF_RC_OK;
+}
+
 int
-diff_output_plain(struct diff_output_info **output_info, FILE *dest,
-		 const struct diff_input_info *info,
-		 const struct diff_result *result)
+diff_output_plain(struct diff_output_info **output_info,
+    FILE *dest, const struct diff_input_info *info,
+    const struct diff_result *result)
 {
 	struct diff_output_info *outinfo = NULL;
+	struct diff_chunk_context cc = {};
+	int atomizer_flags = (result->left->atomizer_flags|
+	    result->right->atomizer_flags);
+	int flags = (result->left->root->diff_flags |
+	    result->right->root->diff_flags);
+	bool force_text = (flags & DIFF_FLAG_FORCE_TEXT_DATA);
+	bool have_binary = (atomizer_flags & DIFF_ATOMIZER_FOUND_BINARY_DATA);
 	int i, rc;
 
 	if (!result)
 		return EINVAL;
 	if (result->rc != DIFF_RC_OK)
 		return result->rc;
-	
+
 	if (output_info) {
 		*output_info = diff_output_info_alloc();
 		if (*output_info == NULL)
-			return errno;
+			return ENOMEM;
 		outinfo = *output_info;
 	}
 
+	if (have_binary && !force_text) {
+		for (i = 0; i < result->chunks.len; i++) {
+			struct diff_chunk *c = &result->chunks.head[i];
+			enum diff_chunk_type t = diff_chunk_type(c);
+
+			if (t != CHUNK_MINUS && t != CHUNK_PLUS)
+				continue;
+
+			fprintf(dest, "Binary files %s and %s differ\n",
+			    diff_output_get_label_left(info),
+			    diff_output_get_label_right(info));
+			break;
+		}
+
+		return DIFF_RC_OK;
+	}
+
 	for (i = 0; i < result->chunks.len; i++) {
-		struct diff_chunk *c = &result->chunks.head[i];
-		if (c->left_count && c->right_count)
-			rc = diff_output_lines(outinfo, dest,
-					  c->solved ? " " : "?",
-					  c->left_start, c->left_count);
-		else if (c->left_count && !c->right_count)
-			rc = diff_output_lines(outinfo, dest,
-					  c->solved ? "-" : "?",
-					  c->left_start, c->left_count);
-		else if (c->right_count && !c->left_count)
-			rc = diff_output_lines(outinfo, dest,
-					  c->solved ? "+" : "?",
-					  c->right_start, c->right_count);
-		if (rc)
+		struct diff_chunk *chunk = &result->chunks.head[i];
+		enum diff_chunk_type t = diff_chunk_type(chunk);
+		struct diff_chunk_context next;
+
+		if (t != CHUNK_MINUS && t != CHUNK_PLUS)
+			continue;
+
+		if (diff_chunk_context_empty(&cc)) {
+			/* Note down the start point, any number of subsequent
+			 * chunks may be joined up to this chunk by being
+			 * directly adjacent. */
+			diff_chunk_context_get(&cc, result, i, 0);
+			continue;
+		}
+
+		/* There already is a previous chunk noted down for being
+		 * printed. Does it join up with this one? */
+		diff_chunk_context_get(&next, result, i, 0);
+
+		if (diff_chunk_contexts_touch(&cc, &next)) {
+			/* This next context touches or overlaps the previous
+			 * one, join. */
+			diff_chunk_contexts_merge(&cc, &next);
+			/* When we merge the last chunk we can end up with one
+			 * hanging chunk and have to come back for it after the
+			 * loop */
+			continue;
+		}
+		rc = output_plain_chunk(outinfo, dest, info, result, &cc);
+		if (rc != DIFF_RC_OK)
 			return rc;
+		cc = next;
 	}
+	if (!diff_chunk_context_empty(&cc))
+		return output_plain_chunk(outinfo, dest, info, result, &cc);
 	return DIFF_RC_OK;
 }