git-mr-create: store mr tags to separate ref

git-mr-create: store mr tags to separate ref

Tags which cannot be changed, like base, commits or commits count, are
stored under new mr ref. Also don't store commit subjects and other
info in tags. Let's get them directly from git in mrlib.load_tags.

Signed-off-by: Frantisek Hrbata <frantisek@hrbata.com>