repository: make check if repo is a worktree more strict

repository: make check if repo is a worktree more strict

To determine if a repository is a worktree or not, we currently check
for the existence of a "gitdir" file inside of the repository's gitdir.
While this is sufficient for non-broken repositories, we have at least
one case of a subtly broken repository where there exists a gitdir file
inside of a gitmodule. This will cause us to misidentify the submodule
as a worktree.

While this is not really a fault of ours, we can do better here by
observing that a repository can only ever be a worktree iff its common
directory and dotgit directory are different. This allows us to make our
check whether a repo is a worktree or not more strict by doing a simple
string comparison of these two directories. This will also allow us to
do the right thing in the above case of a broken repository, as for
submodules these directories will be the same. At the same time, this
allows us to skip the `stat` check for the "gitdir" file for most
repositories.