convert: add a mode where mercurial_sink skips empty revisions. The getchanges function of some converter_source classes can return some false positives. I.e. they sometimes claim that a file "foo" was changed in some revision, even though its contents are still the same. convert_svn is particularly bad, but I think this can also happen with convert_cvs and, at least in theory, with mercurial_source. For regular conversions this is not really a problem - as long as getfile returns the right contents, we'll get a converted revision with the right contents. But when we use --filemap, this could lead to superfluous revisions being converted. Instead of fixing every converter_source, I decided to change mercurial_sink to work around this problem. When --filemap is used, we're interested only in revisions that touch some specific files. If a revision doesn't change any of these files, then we're not interested in it (at least for revisions with a single parent; merges are special). For mercurial_sink, we abuse this property and rollback a commit if the manifest text hasn't changed. This avoids duplicating the logic from localrepo.filecommit to detect unchanged files.

reverting a
changeset 3:107ce1ee2b43 backs out changeset 1:25a1420a55f8
merging with changeset 2:e6c3abc120e7
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
transaction abort!
rollback completed
abort: invalid date: 'should fail' 
transaction abort!
rollback completed
abort: date exceeds 32 bits: 100000000000000000
transaction abort!
rollback completed
abort: impossible time zone offset: 1400000
Sun Jan 15 13:30:00 2006 +0500
Sun Jan 15 13:30:00 2006 -0800
Sat Jul 15 13:30:00 2006 +0500
Sat Jul 15 13:30:00 2006 -0700
Sun Jun 11 00:26:40 2006 -0400
Sat Apr 15 13:30:00 2006 +0200
Sat Apr 15 13:30:00 2006 +0000
Wed Feb 01 13:00:30 2006 -0500
Wed Feb 01 13:00:30 2006 +0000