abspath.h: move absolute path functions from cache.h

This is another step towards letting us remove the include of cache.h in
strbuf.c.  It does mean that we also need to add includes of abspath.h
in a number of C files.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/disk.h

@@ -2,6 +2,7 @@
 #define COMPAT_DISK_H
 
 #include "git-compat-util.h"
+#include "abspath.h"
 #include "gettext.h"
 
 static int get_disk_info(struct strbuf *out)