Check .git dir in clone migration check

When the last part of the repository name is the same as the first part
of it, the old -> new directory migration code is triggered, since a
"top level" directory of the second part's name exists.

In the least-bad case, where the `repos-dir` is separate from the znap
or any other git repository, this triggers an unnecessary invocation of
git, costing a few ms of startup time.

However, the bad case is that some users seem to have set things up so
that their `repos-dir` is a subdirectory of the znap repository. This
causes the `git remote ...` call to succeed, triggering the full
migration, and cloning the repository back into the same place.

Instead of just checking for a directory's existence, we can check for a
git repository's existence fairly cheaply by checking for the `git`
directory.

Fixes #257

functions/.znap.clone.task

@@ -15,7 +15,7 @@ local gitdir=''
 local -P name=${${${url##git@*.*:}%%(|/).git}:t2}
 private new= old=$gitdir/${name:t}
 
-if [[ -d $old ]]; then
+if [[ -d ${old}/.git ]]; then
   if new=$gitdir/${${(M)$(
       git -C $old remote get-url origin 2> /dev/null
   )%%[^/:]##/[^/:]##}%.git}; then