convert: allow the converter_source to say "skip this revision"
If getchanges returns a string, it's assumed to be the id of an
already converted revision. We map the current revision to the same
revision this converted revision was mapped to.
To allow skipping a root revision, getchanges can return the special
string 'hg-convert-skipped-revision' (a.k.a. common.SKIPREV), which
hopefully won't clash with any real id.
The converter_source is responsible for rewriting the parents of the
commit objects to make sure the revision graph makes sense.
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^'!