commit: allow --amend to reuse message from another commit

After tentatively applying a patch from a contributor, you can get a
replacement patch with corrected code and unusable commit log message.
In such a case, this sequence ought to give you an editor based on the
message in the earlier commit, to let you describe an incremental
improvement:

    git reset --hard HEAD^ ;# discard the earlier one
    git am <corrected-patch
    git commit --amend -c HEAD@{1}

Unfortunately, --amend insisted reusing the message from the commit
being amended, ignoring the -c option.  This corrects it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -537,7 +537,7 @@ static int parse_and_validate_options(int argc, const char *argv[],
 		die("Option -m cannot be combined with -c/-C/-F.");
 	if (edit_message)
 		use_message = edit_message;
-	if (amend)
+	if (amend && !use_message)
 		use_message = "HEAD";
 	if (use_message) {
 		unsigned char sha1[20];

t/t7501-commit.sh

@@ -310,4 +310,21 @@ test_expect_success 'same tree (merge and amend merge)' '
 
 '
 
+test_expect_success 'amend using the message from another commit' '
+
+	git reset --hard &&
+	test_tick &&
+	git commit --allow-empty -m "old commit" &&
+	old=$(git rev-parse --verify HEAD) &&
+	test_tick &&
+	git commit --allow-empty -m "new commit" &&
+	new=$(git rev-parse --verify HEAD) &&
+	test_tick &&
+	git commit --allow-empty --amend -C "$old" &&
+	git show --pretty="format:%ad %s" "$old" >expected &&
+	git show --pretty="format:%ad %s" HEAD >actual &&
+	diff -u expected actual
+
+'
+
 test_done