diff/status: print submodule path when looking for changes fails

diff and status run "git status --porcelain" inside each populated
submodule to see if it contains changes (unless told not to do so via
config or command line option). When that fails, e.g. due to a corrupt
submodule .git directory, it just prints "git status --porcelain failed"
or "Could not run git status --porcelain" without giving the user a clue
where that happened.

Add '"in submodule %s", path' to these error strings to tell the user
where exactly the problem occurred.

Reported-by: Seth Robertson <in-gitvger@baka.org>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -148,13 +148,13 @@ int is_submodule_modified(const char *path)
 	cp.no_stdin = 1;
 	cp.out = -1;
 	if (start_command(&cp))
-		die("Could not run git status --porcelain");
+		die("Could not run 'git status --porcelain' in submodule %s", path);
 
 	len = strbuf_read(&buf, cp.out, 1024);
 	close(cp.out);
 
 	if (finish_command(&cp))
-		die("git status --porcelain failed");
+		die("'git status --porcelain' failed in submodule %s", path);
 
 	free(env[0]);
 	free(env[1]);