Josh Rickmar reported a bug on #gameoftrees IRC that ended up being
shared with diff3 code inherited from OpenBSD. See here for details
and patches for the OpenBSD base tools:
https://marc.info/?l=openbsd-tech&m=161825519000776&w=2

This is the corresponding fix for the diff3 code in Got.
It makes the problematic cherrypick which Josh identified succeed.

ok?

diff 6c19a3dcfa5f03ec889d629dded9ff4dfa9f6237 /home/stsp/src/got
blob - 6343f4f287533c9a4793e0e93a169a6bc59a5d23
file + lib/diff3.c
--- lib/diff3.c
+++ lib/diff3.c
@@ -885,10 +885,12 @@ duplicate(int *dpl, int j, struct line_range *r1, stru
 	for (nline = 0; nline < r1->to - r1->from; nline++) {
 		do {
 			c = getc(d3s->fp[0]);
-			if (c == EOF)
-				return got_ferror(d3s->fp[0], GOT_ERR_EOF);
 			d = getc(d3s->fp[1]);
-			if (d == EOF)
+			if (c == EOF && d == EOF)
+				break;
+			else if (c == EOF)
+				return got_ferror(d3s->fp[0], GOT_ERR_EOF);
+			else if (d == EOF)
 				return got_ferror(d3s->fp[1], GOT_ERR_EOF);
 			nchar++;
 			if (c != d) {