This patch adds a very simple e2e test of having a working copy shared
with Git. The test initially failed on Windows. The symptom was that
the "master" branch did not get updated when we create a commit using
`jj`. That suggested that we didn't correctly detect that the working
copy was shared. After a lot of troubleshooting, I think I mostly
understand what we going on here (thanks to @arxanas for suggesting
https://github.com/mxschmitt/action-tmate). The path we get from
`git2::Repository::workdir()` seems to not be canonicalized in the
same way as `std::fs::canonicalize()` canonicalizes. Specifically, it
does not have the "\\?\" prefix we get from that function. I suppose
that's because libgit2 is a C library and canonicalizes the path using
some other system call.
