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.
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1