reset: suggest what to do upon "git reset --mixed <paths>"

When you call "git reset --mixed <paths>" git will warn that using mixed
with paths is deprecated:

    warning: --mixed option is deprecated with paths.

That doesn't tell the user what he should use instead. Expand on the
warning and tell the user to just omit --mixed:

    warning: --mixed with paths is deprecated; use 'git reset -- <paths>' instead

The exact wording of the warning was suggested by Jonathan Nieder.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/reset.c

@@ -318,7 +318,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	 * affecting the working tree nor HEAD. */
 	if (i < argc) {
 		if (reset_type == MIXED)
-			warning("--mixed option is deprecated with paths.");
+			warning("--mixed with paths is deprecated; use 'git reset -- <paths>' instead.");
 		else if (reset_type != NONE)
 			die("Cannot do %s reset with paths.",
 					reset_type_names[reset_type]);