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 foo
changeset 2:4d9e78aaceee backs out changeset 1:b515023e500e
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
resolving manifests
 overwrite None partial False
 ancestor bbd179dfa0a7 local 71766447bdbb+ remote 4d9e78aaceee
  searching for copies back to rev 1
  unmatched files in local:
   bar
 foo: remote is newer -> g
getting foo
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
n   0         -2 unset             foo
M foo
c6fc755d7e68f49f880599da29f15add41f42f5a 644 foo
   rev    offset  length   base linkrev nodeid       p1           p2
     0         0       5      0       0 2ed2a3912a0b 000000000000 000000000000
     1         5       9      1       1 6f4310b00b9a 2ed2a3912a0b 000000000000
     2        14       5      2       2 c6fc755d7e68 6f4310b00b9a 000000000000