Found a bug in git-annex-shell where verbose messages would sometimes make
it output things git-annex didn't expect.

While fixing that, I wanted to add a test case, but the test suite actually
does not test git-annex-shell at all. It would need to ssh, which test
suites should not do. So, I took a detour..

Support for `GIT_SSH` and `GIT_SSH_COMMAND` has been requested before for
various reasons. So I implemented that, which took 4 hours. (With one
little possible compatibility caveat, since git-annex needs to pass the -n
parameter to ssh sometimes, and git's interface doesn't allow for such a
parameter.)

Now the test suite can use those environment variables to make mock ssh
remotes be accessed using local `sh` instead of `ssh`.

Today's work was sponsored by Trenton Cronholm on Patreon.