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>

abspath.c

@@ -1,4 +1,6 @@
-#include "cache.h"
+#include "git-compat-util.h"
+#include "abspath.h"
+#include "strbuf.h"
 
 /*
  * Do not use this for inspecting *tracked* content.  When path is a