Merge branch 'tr/protect-low-3-fds'

When "git" is spawned in such a way that any of the low 3 file
descriptors is closed, our first open() may yield file descriptor 2,
and writing error message to it would screw things up in a big way.

* tr/protect-low-3-fds:
  git: ensure 0/1/2 are open in main()
  daemon/shell: refactor redirection of 0/1/2 from /dev/null

cache.h

@@ -425,6 +425,8 @@ extern int path_inside_repo(const char *prefix, const char *path);
 extern int set_git_dir_init(const char *git_dir, const char *real_git_dir, int);
 extern int init_db(const char *template_dir, unsigned int flags);
 
+extern void sanitize_stdfds(void);
+
 #define alloc_nr(x) (((x)+16)*3/2)
 
 /*