tests/test-586

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.

#!/bin/sh
# a test for issue586

hg init a
cd a
echo a > a
hg ci -Ama

hg init ../b
cd ../b
echo b > b
hg ci -Amb

hg pull -f ../a
hg merge
hg rm -f a
hg ci -Amc

hg st -A