the primary purpose of this change is making it work with new-style
submodules (1.7+) and git-worktree (2.5+). this means resolving '.git'
and 'commondir' files.

we avoid calling git commands, because a) it's slow on windows, b) some
of them are unavailable in older git versions and it's tedious to deal
with that and c) their output is not necessarily optimal (not always
absolute paths).

we also don't use relative paths for the hook locations any more, as
that's too tedious to deal with.

Change-Id: Ie341e748e56d6bef40856e4a49ac368850028d83
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>