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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6:755d1e0d79e9 

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5:9ce2ce29723a 3:a3e00c7dbf11 4:bb4475edb621 

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4:bb4475edb621 1:5d953a1917d1 

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3:a3e00c7dbf11 

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2:befc7d89d081 

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1:5d953a1917d1 

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0:837088b6e1d9 

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should work with tag/branch/node/rev

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tip^: 5

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default^: 5

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755d1e0d79e9^: 5

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6^: 5

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some random lookups

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6^^: 3

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6^^^: 2

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6^^^^: 1

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6^^^^^: 0

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6^^^^^^: -1

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6^1: 5

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6^2: abort: unknown revision '6^2'!

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6^^2: 4

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6^1^2: 4

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6^^3: abort: unknown revision '6^^3'!

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6~: abort: unknown revision '6~'!

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6~1: 5

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6~2: 3

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6~3: 2

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6~4: 1

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6~5: 0

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6~42: -1

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6~1^2: 4

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6~1^2~2: 0

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with a tag "6^" pointing to rev 1

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6^: 1

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6^1: 5

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6~1: 5

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6^^: 3

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with a tag "foo^bar" pointing to rev 2

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foo^bar: 2

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foo^bar^: abort: unknown revision 'foo^bar^'!