The new wording below is a bit longer but I hope it will be less
confusing to new users, especially if they do not know Git very well.
It is based in part on how the -n option is documented in the manual.

before:

got: merge is a fast-forward; this is incompatible with got merge -n

after:

got: there are no changes to merge since refs/heads/newbranch is \
  already based on refs/heads/master; merge cannot be interrupted \
  for amending; -n: option cannot be used

I also considered dropping this check entirely. But since -n is not
the default behaviour it makes sense to stop the operation and let
the user reconsider their approach.

-----------------------------------------------
 reword user-facing error message which mentions "fast-forward"
 
 For user-facing messages it is better to avoid technical jargon like
 this and instead spell out what the fast-forward situation implies: that
 one branch is already based on another.
 
diff b73055ebbea59457c1aef2cf7c473ea2898b434b 15fdc4b7bab4dbe54aa6c2a8c401c62882f70a0a
commit - b73055ebbea59457c1aef2cf7c473ea2898b434b
commit + 15fdc4b7bab4dbe54aa6c2a8c401c62882f70a0a
blob - ea8a08baed68affba73cafa577c3125e9756466a
blob + 61dfa9d2bbab0c9bcc68905af78e9f8c9270bfbe
--- got/got.c
+++ got/got.c
@@ -13319,9 +13319,11 @@ cmd_merge(int argc, char *argv[])
 		if (yca_id && got_object_id_cmp(wt_branch_tip, yca_id) == 0) {
 			struct got_pathlist_head paths;
 			if (interrupt_merge) {
-				error = got_error_msg(GOT_ERR_SAME_BRANCH,
-				    "merge is a fast-forward; this is "
-				    "incompatible with got merge -n");
+				error = got_error_fmt(GOT_ERR_BAD_OPTION,
+				    "there are no changes to merge since %s "
+				    "is already based on %s; merge cannot be "
+				    "interrupted for amending; -n",
+				    branch_name, got_ref_get_name(wt_branch));
 				goto done;
 			}
 			printf("Forwarding %s to %s\n",
blob - c7c5ce296704a7d0c3345092eccebd5837fcd453
blob + 6ca6d041c1b17b5198707cc33b19f9fefccf07e0
--- include/got_error.h
+++ include/got_error.h
@@ -171,7 +171,7 @@
 #define GOT_ERR_BAD_TAG_SIGNATURE 154
 #define GOT_ERR_VERIFY_TAG_SIGNATURE 155
 #define GOT_ERR_SIGNING_TAG	156
-/* 157 is currently unused */
+#define GOT_ERR_BAD_OPTION 157
 #define GOT_ERR_BAD_QUERYSTRING	158
 #define GOT_ERR_INTEGRATE_BRANCH 159
 #define GOT_ERR_BAD_REQUEST	160
blob - 765efe263e09f28dfd6b925eaa64ee0ffe226a3b
blob + 14daf3a57c9be57d4f707ce25da1a357e09ada88
--- lib/error.c
+++ lib/error.c
@@ -217,6 +217,7 @@ static const struct got_error got_errors[] = {
 	{ GOT_ERR_BAD_TAG_SIGNATURE, "invalid tag signature" },
 	{ GOT_ERR_VERIFY_TAG_SIGNATURE, "cannot verify signature" },
 	{ GOT_ERR_SIGNING_TAG, "unable to sign tag" },
+	{ GOT_ERR_BAD_OPTION, "option cannot be used" },
 	{ GOT_ERR_BAD_QUERYSTRING, "invalid query string" },
 	{ GOT_ERR_INTEGRATE_BRANCH, "will not integrate into a reference "
 	    "outside the \"refs/heads/\" reference namespace" },
blob - 92b590aa2ef2958738fb2d4e8419cb879fc9bf5c
blob + cd813c681b68090d962e708e8d375740e5ac421c
--- regress/cmdline/merge.sh
+++ regress/cmdline/merge.sh
@@ -415,9 +415,16 @@ test_merge_forward() {
 		test_done "$testroot" "1"
 		return 1
 	fi
-	echo -n "got: merge is a fast-forward; " > $testroot/stderr.expected 
-	echo "this is incompatible with got merge -n" \
+
+	echo -n "got: there are no changes to merge since " \
+		> $testroot/stderr.expected
+	echo -n "refs/heads/newbranch is already based on " \
 		>> $testroot/stderr.expected
+	echo -n "refs/heads/master; merge cannot be interrupted " \
+		>> $testroot/stderr.expected
+	echo "for amending; -n: option cannot be used" \
+		>> $testroot/stderr.expected
+
 	cmp -s $testroot/stderr.expected $testroot/stderr
 	ret=$?
 	if [ $ret -ne 0 ]; then