[PATCH v3 1/4] environment.c: fix potential segfault by get_git_common_dir()

setup_git_env() must be called before this function to initialize
git_common_dir so that it returns a non NULL string. And it must return
a non NULL string or segfault can happen because all callers expect so.

Normally if somebody has called get_git_dir(), or set_git_dir() then
setup_git_env() is already called. But if you do setup_git_directory()
at top dir (which skips set_git_dir) and never call get_git_dir, you'll
get NULL here.

test-ref-store.c will hit this problem because it's very lightweight,
just enough initialization to exercise refs code, and get_git_dir() will
never be called until get_worktrees() is, which uses get_git_common_dir().
---
 environment.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/environment.c b/environment.c
index 42dc3106d2..2986ee7200 100644
--- a/environment.c
+++ b/environment.c

@@ -214,6 +214,8 @@ const char *get_git_dir(void)
 
 const char *get_git_common_dir(void)
 {
+	if (!git_dir)
+		setup_git_env();
 	return git_common_dir;
 }
 

-- 
2.11.0.157.gd943d85