fmt-merge-msg: use branch.$name.description

This teaches "merge --log" and fmt-merge-msg to use branch description
information when merging a local topic branch into the mainline. The
description goes between the branch name label and the list of commit
titles.

The refactoring to share the common configuration parsing between
merge and fmt-merge-msg needs to be made into a separate patch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

fmt-merge-msg.h

@@ -0,0 +1,7 @@
+#ifndef FMT_MERGE_MSG_H
+#define FMT_MERGE_MSG_H
+
+extern int merge_log_config;
+extern int fmt_merge_msg_config(const char *key, const char *value, void *cb);
+
+#endif /* FMT_MERGE_MSG_H */