commit-tree: add the commit.gpgsign option to sign all commits

If you want to GPG sign all your commits, you have to add the -S option
all the time. The commit.gpgsign config option allows to sign all
commits automatically.

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/merge.c b/builtin/merge.c
index 02a69c1..fea2724 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -604,6 +604,9 @@
 	} else if (!strcmp(k, "merge.defaulttoupstream")) {
 		default_to_upstream = git_config_bool(k, v);
 		return 0;
+	} else if (!strcmp(k, "commit.gpgsign")) {
+		sign_commit = git_config_bool(k, v) ? "" : NULL;
+		return 0;
 	}
 
 	status = fmt_merge_msg_config(k, v, cb);