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.

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running: init test1

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result: None

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running: add foo

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result: None

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running: commit -m commit1 -d 2000-01-01 foo

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result: None

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running: commit -m commit2 -d 2000-01-02 foo

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result: None

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running: log -r 0

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changeset:   0:0e4634943879

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user:        test

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date:        Sat Jan 01 00:00:00 2000 +0000

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summary:     commit1

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result: None

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running: log -r tip

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changeset:   1:45589e459b2e

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tag:         tip

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user:        test

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date:        Sun Jan 02 00:00:00 2000 +0000

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summary:     commit2

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result: None