replace: remove signature when using --graft

It could be misleading to keep a signature in a
replacement commit, so let's remove it.

Note that there should probably be a way to sign
the replacement commit created when using --graft,
but this can be dealt with in another commit or
patch series.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/replace.c b/builtin/replace.c
index 7459359..52f73ce 100644
--- a/builtin/replace.c
+++ b/builtin/replace.c
@@ -344,6 +344,11 @@
 
 	replace_parents(&buf, argc - 1, &argv[1]);
 
+	if (remove_signature(&buf)) {
+		warning(_("the original commit '%s' has a gpg signature."), old_ref);
+		warning(_("the signature will be removed in the replacement commit!"));
+	}
+
 	if (write_sha1_file(buf.buf, buf.len, commit_type, new))
 		die(_("could not write replacement commit for: '%s'"), old_ref);