maintenance: fix incorrect `maintenance.repo` path with bare repository

maintenance: fix incorrect `maintenance.repo` path with bare repository

The periodic maintenance tasks configured by `git maintenance start`
invoke `git for-each-repo` to run `git maintenance run` on each path
specified by the multi-value global configuration variable
`maintenance.repo`. Because `git for-each-repo` will likely be run
outside of the repositories which require periodic maintenance, it is
mandatory that the repository paths specified by `maintenance.repo` are
absolute.

Unfortunately, however, `git maintenance register` does nothing to
ensure that the paths it assigns to `maintenance.repo` are indeed
absolute, and may in fact -- especially in the case of a bare repository
-- assign a relative path to `maintenance.repo` instead. Fix this
problem by converting all paths to absolute before assigning them to
`maintenance.repo`.

While at it, also fix `git maintenance unregister` to convert paths to
absolute, as well, in order to ensure that it can correctly remove from
`maintenance.repo` a path assigned via `git maintenance register`.

Reported-by: Clement Moyroud <clement.moyroud@gmail.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>