parse-options: disable option abbreviation with PARSE_OPT_KEEP_UNKNOWN

parse-options can unambiguously find an abbreviation only if it sees
all available options. This is usually the case when you use
parse_options(). But there are other callers like blame or shortlog
which uses parse_options_start() in combination with a custom option
parser, like rev-list. parse-options cannot see all options in this
case and will get abbrev detection wrong. Disable it.

t7800 needs update because --symlink no longer expands to --symlinks
and will be passed down to git-diff, which will not recognize it. I
still think this is the correct thing to do. But if --symlink has been
actually used in the wild, we would just add an option alias for it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.c

@@ -266,7 +266,8 @@ again:
 		}
 		if (!rest) {
 			/* abbreviated? */
-			if (!strncmp(long_name, arg, arg_end - arg)) {
+			if (!(p->flags & PARSE_OPT_KEEP_UNKNOWN) &&
+			    !strncmp(long_name, arg, arg_end - arg)) {
 is_abbreviated:
 				if (abbrev_option) {
 					/*

t/t7800-difftool.sh

@@ -546,7 +546,7 @@ do
 done >actual
 EOF
 
-test_expect_success SYMLINKS 'difftool --dir-diff --symlink without unstaged changes' '
+test_expect_success SYMLINKS 'difftool --dir-diff --symlinks without unstaged changes' '
 	cat >expect <<-EOF &&
 	file
 	$PWD/file
@@ -555,7 +555,7 @@ test_expect_success SYMLINKS 'difftool --dir-diff --symlink without unstaged cha
 	sub/sub
 	$PWD/sub/sub
 	EOF
-	git difftool --dir-diff --symlink \
+	git difftool --dir-diff --symlinks \
 		--extcmd "./.git/CHECK_SYMLINKS" branch HEAD &&
 	test_cmp expect actual
 '