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.

% manifest of p2:
bar
foo

% manifest of p1:
foo
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
% this should not mention bar:
changeset:   3:ef2fc9b4a51b
tag:         tip
parent:      2:ed1b79f46b9a
parent:      1:d394a8db219b
user:        test
date:        Thu Jan 01 00:00:00 1970 +0000
description:
merge


   rev    offset  length   base linkrev nodeid       p1           p2
     0         0       5      0       1 b004912a8510 000000000000 000000000000