share: add --relative flag to store a relative path to the source
Storing a relative path the source repository is useful when exporting
repositories over the network or when they're located on external
drives where the mountpoint isn't always fixed.
Currently, Mercurial interprets paths in `.hg/shared` relative to
$PWD. I suspect this is very much unintentional, and you have to
manually edit `.hg/shared` in order to trigger this behaviour.
However, on the off chance that someone might rely on it, I added a
new capability called 'relshared'. In addition, this makes earlier
versions of Mercurial fail with a graceful error.
I should note that I haven't tested this patch on Windows.