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.

adding a
? a
adding a
A a
A a
? b
A a
A b
% should fail
b already tracked!
A a
A b
% should fail
a already tracked!
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
warning: conflicts during merge.
merging a
merging a failed!
0 files updated, 0 files merged, 0 files removed, 1 files unresolved
There are unresolved merges, you can redo the full merge using:
  hg update -C 2
  hg merge 1
M a
% should fail
a already tracked!
M a
% issue683
R a
M a