path: create path.h

Move all path related declarations from cache.h to a new path.h header
file.  This makes cache.h smaller and makes it easier to add new path
related functions.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -8,6 +8,7 @@
 #include "dir.h"
 #include "worktree.h"
 #include "submodule-config.h"
+#include "path.h"
 
 static int get_st_mode_bits(const char *path, int *mode)
 {