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.

--- a/hgext/convert/common.py
+++ b/hgext/convert/common.py
@@ -167,3 +167,13 @@ class converter_sink(object):
         pbranch: branch name of parent commit
         parents: destination revisions of parent"""
         pass
+
+    def setfilemapmode(self, active):
+        """Tell the destination that we're using a filemap
+
+        Some converter_sources (svn in particular) can claim that a file
+        was changed in a revision, even if there was no change.  This method
+        tells the destination that we're using a filemap and that it should
+        filter empty revisions.
+        """
+        pass