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.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).