better support for git worktree

[andreineculau]

running git worktree add some-branch some-branch in a transcrypted repository fails because the "common" config will point to a .git/worktrees/some-branch/transcrypt instead of .git/worktrees/transcrypt

this fix is half-baked because it then makes it impossible to activate transcrypt only inside 1 worktree, but overall it makes sense

[jmurty]

Thanks for this, I have extended your change so transcrypt (and tests) uses the --git-common-dir option instead of --git-dir in all places.

Does this change make sense? And does it get transcrypt closer to working properly with worktrees (with the caveat you mentioned)?

[andreineculau]

Looks good 👍

[djmaze]

Works for me, nice!