git status: show relative paths when run in a subdirectory

To show the relative paths, the function formerly called quote_crlf()
(now called quote_path()) takes the prefix as an additional argument.

While at it, the static buffers were replaced by strbufs.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-runstatus.c

@@ -14,6 +14,7 @@ int cmd_runstatus(int argc, const char **argv, const char *prefix)
 
 	git_config(git_status_config);
 	wt_status_prepare(&s);
+	s.prefix = prefix;
 
 	for (i = 1; i < argc; i++) {
 		if (!strcmp(argv[i], "--color"))