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
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% expect ssl error
pushing to http://localhost:20059/
searching for changes
ssl required
% expect authorization error
pushing to http://localhost:20059/
searching for changes
push not authorized
% expect authorization error: must have authorized user
pushing to http://localhost:20059/
searching for changes
push not authorized
% expect success
pushing to http://localhost:20059/
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
rolling back last transaction
changegroup hook: HG_NODE=ba677d0156c1196c1a699fa53f390dcfc3ce3872 HG_SOURCE=serve HG_URL=remote:http 
% expect authorization error: all users denied
pushing to http://localhost:20059/
searching for changes
push not authorized
% expect authorization error: some users denied, users must be authenticated
pushing to http://localhost:20059/
searching for changes
push not authorized