[RFC PATCH] rev-parse: fix segfault with missing --path-format argument

["Wolfgang Müller" <wolf@oriole.systems>]

Calling "git rev-parse --path-format" without an argument segfaults
instead of giving an error message. Commit fac60b8925 (rev-parse: add
option for absolute or relative path formatting, 2020-12-13) added the
argument parsing code but forgot to handle NULL.

Returning an error makes sense here because there is no default value we
could use. Add a test case to verify.

Signed-off-by: Wolfgang Müller <wolf@oriole.systems>
---
Since this is my first contribution to the project and I'm still
learning the ropes, I left this patch as RFC.

For a bit of background information, I ran into this expecting the
following to work:

	git rev-parse --path-format relative --show-toplevel

I'm unsure how many git subcommands specifically require "=" between the
option and the argument, but before now I always expected things to
"just work" when leaving it out.

This fix is based on maint.

Thanks for your time and attention.

 builtin/rev-parse.c  | 2 ++
 t/t1500-rev-parse.sh | 4 ++++
 2 files changed, 6 insertions(+)

diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index 85bad9052e..7af8dab8bc 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -759,6 +759,8 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				continue;
 			}
 			if (opt_with_value(arg, "--path-format", &arg)) {
+				if (!arg)
+					die("--path-format requires an argument");
 				if (!strcmp(arg, "absolute")) {
 					format = FORMAT_CANONICAL;
 				} else if (!strcmp(arg, "relative")) {
diff --git a/t/t1500-rev-parse.sh b/t/t1500-rev-parse.sh
index deae916707..a1a8ce5265 100755
--- a/t/t1500-rev-parse.sh
+++ b/t/t1500-rev-parse.sh
@@ -146,6 +146,10 @@ test_expect_success '--path-format can change in the middle of the command line'
 	test_cmp expect actual
 '
 
+test_expect_success '--path-format does not segfault without an argument' '
+	test_must_fail git rev-parse --path-format --show-toplevel
+'
+
 test_expect_success 'git-common-dir from worktree root' '
 	echo .git >expect &&
 	git rev-parse --git-common-dir >actual &&
-- 
2.31.1