abspath: convert absolute_path() to strbuf

Move most of the code of absolute_path() into the new function
strbuf_add_absolute_path() and in the process transform it to use
struct strbuf and xgetcwd() instead of a PATH_MAX-sized buffer,
which can be too small on some file systems.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/strbuf.h b/strbuf.h
index 23b16c6..455826c 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -179,6 +179,8 @@
 				    int reserved);
 extern void strbuf_humanise_bytes(struct strbuf *buf, off_t bytes);
 
+extern void strbuf_add_absolute_path(struct strbuf *sb, const char *path);
+
 __attribute__((format (printf,1,2)))
 extern int printf_ln(const char *fmt, ...);
 __attribute__((format (printf,2,3)))