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.
$ echo 'raise Exception("bit bucket overflow")' > badext.py
$ abspath=`pwd`/badext.py
$ cat <<EOF >> $HGRCPATH
> [extensions]
> gpg =
> hgext.gpg =
> badext = $abspath
> badext2 =
> EOF
$ hg -q help help
*** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
*** failed to import extension badext2: No module named badext2
hg help [-ec] [TOPIC]
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