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.

<table class="logEntry parity#parity#">
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  <th class="age">#date|age# ago:</th>
  <th class="firstline"><a href="#url#rev/#node|short#{sessionvars%urlparameter}">#desc|strip|firstline|escape#</a></th>
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 <tr>
  <th class="revision">revision #filerev#:</td>
  <td class="node">
   <a href="#url#file/#node|short#/#file|urlescape#{sessionvars%urlparameter}">#node|short#</a>
   <a href="#url#diff/#node|short#/#file|urlescape#{sessionvars%urlparameter}">(diff)</a>
   <a href="#url#annotate/#node|short#/#file|urlescape#{sessionvars%urlparameter}">(annotate)</a>
  </td>
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 #rename%filelogrename#
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  <th class="author">author:</th>
  <td class="author">#author|obfuscate#</td>
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 <tr>
  <th class="date">date:</th>
  <td class="date">#date|date#</td>
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</table>