[PATCH 11/16] Do not read .git/info/attributes if there is no repository

This one is difficult to demonstrate by tests. So I'll describe the
call flow instead.

Say we do "git grep --no-index --show-function foo -- bar" in no
repository. The call flow would be:
 - ...
 - cmd_grep
 - grep_directory
 - grep_file
 - grep_buffer
 - grep_buffer_1
 - userdiff_find_by_path
 - git_checkattr("diff")
 - bootstrap_attr_stack
 - git_path("info/attributes")

Because no repository is found, git_dir in environment.c would be
NULL. When it reaches git_path("info/attributes"), it will
automatically set git_dir to ".git". If we have $(cwd)/.git/info/attributes
then that file will be read, no matter $(cwd)/.git is a valid
repository.

This bug is hard to be exposed, because after git_checkattr("diff")
finds something (wrong). It will look up for diff drivers from
.git/config. If we do thing correctly, .git/config will not be read,
thus no diff drivers, no visible impact. If .git/config is read,
git_dir must be set already, all this would not happen.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
---
 attr.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/attr.c b/attr.c
index f5346ed..f1362de 100644
--- a/attr.c
+++ b/attr.c

@@ -480,7 +480,10 @@ static void bootstrap_attr_stack(void)
 			debug_push(elem);
 		}
 
-		elem = read_attr_from_file(git_path(INFOATTRIBUTES_FILE), 1);
+		if (!startup_info || startup_info->have_repository)
+			elem = read_attr_from_file(git_path(INFOATTRIBUTES_FILE), 1);
+		else
+			elem = NULL;
 		if (!elem)
 			elem = xcalloc(1, sizeof(*elem));
 		elem->origin = NULL;

-- 
1.7.0.1.384.g6abcaa