[PATCH v2 2/3] setup: do not use invalid `repository_format`

["Martin Ågren" <martin.agren@gmail.com>]

If `read_repository_format()` encounters an error, `format->version`
will be -1 and all other fields of `format` will be undefined. However,
in `setup_git_directory_gently()`, we use `repo_fmt.hash_algo`
regardless of the value of `repo_fmt.version`.

This can be observed by adding this to the end of
`read_repository_format()`:

	if (format->version == -1)
		format->hash_algo = 0; /* no-one should peek at this! */

This causes, e.g., "git branch -m q q2 without config should succeed" in
t3200 to fail with "fatal: Failed to resolve HEAD as a valid ref."
because it has moved .git/config out of the way and is now trying to use
a bad hash algorithm.

Check that `version` is non-negative before using `hash_algo`.

This patch adds no tests, but do note that if we skip this patch, the
next patch would cause existing tests to fail as outlined above.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
---
 setup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/setup.c b/setup.c
index bb633942bb..4d3d67c50b 100644
--- a/setup.c
+++ b/setup.c
@@ -1140,7 +1140,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 				gitdir = DEFAULT_GIT_DIR_ENVIRONMENT;
 			setup_git_env(gitdir);
 		}
-		if (startup_info->have_repository)
+		if (startup_info->have_repository && repo_fmt.version > -1)
 			repo_set_hash_algo(the_repository, repo_fmt.hash_algo);
 	}
 
-- 
2.20.1