am: configure gpg at startup

The new builtin am ignores the user.signingkey variable: gpg is being
called with the committer details as the key ID, which may not be
correct. git_gpg_config is responsible for handling that variable and is
expected to be called on initialization by any modules that use gpg.

Signed-off-by: Renee Margaret McConahy <nepella@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -2125,6 +2125,17 @@ enum resume_mode {
 	RESUME_ABORT
 };
 
+static int git_am_config(const char *k, const char *v, void *cb)
+{
+	int status;
+
+	status = git_gpg_config(k, v, NULL);
+	if (status)
+		return status;
+
+	return git_default_config(k, v, NULL);
+}
+
 int cmd_am(int argc, const char **argv, const char *prefix)
 {
 	struct am_state state;
@@ -2223,7 +2234,7 @@ int cmd_am(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
-	git_config(git_default_config, NULL);
+	git_config(git_am_config, NULL);
 
 	am_state_init(&state, git_path("rebase-apply"));