share: fix source repo lookup on Windows
The stored path contains platform specific separators, so splitting on '/.hg'
returned the string unmodified on Windows. The source was then looked for in
$source/.hg/.hg, which obviously fails. This caused cascading errors in
test-share.t relating to the recent bookmark support.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.