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.
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..