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.

6:755d1e0d79e9 
5:9ce2ce29723a 3:a3e00c7dbf11 4:bb4475edb621 
4:bb4475edb621 1:5d953a1917d1 
3:a3e00c7dbf11 
2:befc7d89d081 
1:5d953a1917d1 
0:837088b6e1d9 

should work with tag/branch/node/rev
tip^: 5
default^: 5
755d1e0d79e9^: 5
6^: 5

some random lookups
6^^: 3
6^^^: 2
6^^^^: 1
6^^^^^: 0
6^^^^^^: -1
6^1: 5
6^2: abort: unknown revision '6^2'!
6^^2: 4
6^1^2: 4
6^^3: abort: unknown revision '6^^3'!
6~: abort: unknown revision '6~'!
6~1: 5
6~2: 3
6~3: 2
6~4: 1
6~5: 0
6~42: -1
6~1^2: 4
6~1^2~2: 0

with a tag "6^" pointing to rev 1
6^: 1
6^1: 5
6~1: 5
6^^: 3

with a tag "foo^bar" pointing to rev 2
foo^bar: 2
foo^bar^: abort: unknown revision 'foo^bar^'!