notmuch restore: Don't waste time with messages with unchanged tags.

notmuch restore: Don't waste time with messages with unchanged tags.

It's a simple optimization to look at a message and check that the
existing tags are actually different than the tags we are setting
before we do anything.

For my mail store this takes a "notmuch restore" that does nothing
from about 10 minutes down to 1 minute, so there's a significant
speedup here.