abspath: add absolute_pathdup()

Add a function that returns a buffer containing the absolute path of its
argument and a semantic patch for its intended use.  It avoids an extra
string copy to a static buffer.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

abspath.c

@@ -152,6 +152,13 @@ const char *absolute_path(const char *path)
 	return sb.buf;
 }
 
+char *absolute_pathdup(const char *path)
+{
+	struct strbuf sb = STRBUF_INIT;
+	strbuf_add_absolute_path(&sb, path);
+	return strbuf_detach(&sb, NULL);
+}
+
 /*
  * Unlike prefix_path, this should be used if the named file does
  * not have to interact with index entry; i.e. name of a random file