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.

resolving manifests
 overwrite None partial False
 ancestor 310fd17130da local 2092631ce82b+ remote 7731dad1c2b9
  searching for copies back to rev 1
  unmatched files in other:
   bar
  all copies found (* = to merge, ! = divergent):
   bar -> foo *
  checking for directory renames
 foo: versions differ -> m
 foo: remote copied to bar -> m
copying foo to bar
merging foo and bar
my foo@2092631ce82b+ other bar@7731dad1c2b9 ancestor foo@310fd17130da
merging foo
my foo@2092631ce82b+ other foo@7731dad1c2b9 ancestor foo@310fd17130da
0 files updated, 2 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
-- foo --
line 0
line 1
line 2-1
-- bar --
line 0
line 1
line 2-2