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.

checkout
2 files updated, 0 files merged, 2 files removed, 0 files unresolved
merge
resolving manifests
 overwrite None partial False
 ancestor af1939970a1c local f26ec4fc3fa3+ remote 8e765a822af2
  searching for copies back to rev 1
  unmatched files in local:
   c2
  unmatched files in other:
   b
   b2
  all copies found (* = to merge, ! = divergent):
   c2 -> a2 
   b -> a *
   b2 -> a2 
  checking for directory renames
 a2: divergent renames -> dr
 a: remote moved to b -> m
 b2: remote created -> g
copying a to b
merging a and b
my a@f26ec4fc3fa3+ other b@8e765a822af2 ancestor a@af1939970a1c
removing a
warning: detected divergent renames of a2 to:
 c2
 b2
getting b2
1 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
M b
  a
M b2
R a
C c2
blahblah
   rev    offset  length   base linkrev nodeid       p1           p2
     0         0      67      0       1 dc51707dfc98 000000000000 000000000000
     1        67      72      1       3 b2494a44f0a9 000000000000 dc51707dfc98
b renamed from a:dd03b83622e78778b403775d0d074b9ac7387a66