merge: fix unnecessary rename merges on linear update (issue631)
If one side's revision is identical to the ancestor, we skip the rest
of the copy detection logic.
tag: handle .hgtags and .hg/localtags with missing final newline (issue 601)
This also fixes an asymmetry bug in which we called the pretag hook if we
were going to create a local tag, but didn't call the tag hook afterwards.