abspath: move related functions to abspath

Move abspath-related functions from strbuf.[ch] to abspath.[ch] so that
strbuf is focused on string manipulation routines with minimal
dependencies.

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tempfile.c

@@ -43,6 +43,7 @@
  */
 
 #include "git-compat-util.h"
+#include "abspath.h"
 #include "path.h"
 #include "tempfile.h"
 #include "sigchain.h"