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
# missing arg
hg cat: invalid arguments
hg cat [OPTION]... FILE...

output the current or given revision of files

    Print the specified files as they were at the given revision.
    If no revision is given, the parent of the working directory is used,
    or tip if no revision is checked out.

    Output may be to a file, in which case the name of the file is
    given using a format string.  The formatting rules are the same as
    for the export command, with the following additions:

    %s   basename of file being printed
    %d   dirname of file being printed, or '.' if in repo root
    %p   root-relative path name of file being printed

options:

 -o --output   print output to file with formatted name
 -r --rev      print the given revision
 -I --include  include names matching the given patterns
 -X --exclude  exclude names matching the given patterns

use "hg -v help cat" to show global options
% [defaults]
a
a
% no repo
abort: There is no Mercurial repository here (.hg not found)!