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/commit.c b/builtin/commit.c
index 6ab4605..cffddf2 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1406,6 +1406,10 @@
 	}
 	if (!strcmp(k, "commit.cleanup"))
 		return git_config_string(&cleanup_arg, k, v);
+	if (!strcmp(k, "commit.gpgsign")) {
+		sign_commit = git_config_bool(k, v) ? "" : NULL;
+		return 0;
+	}
 
 	status = git_gpg_config(k, v, NULL);
 	if (status)