Add workaround for git worktree

[steinybot]

Take 2.

Turns out it is actually quite easy to detect if the current workspace is a linked workspace because .git is a file instead of a directory.

[sideeffffect]

Could this even be upstreamed to sbt-git?

[steinybot]

Yep, I've got a PR there sbt/sbt-git#243. Dunno how long it'll take to get in there.

Not sure if you mind having a temporary workaround. It's a bit fiddly having to reapply this change every time I switch branches.

[fwbrasil]

LGTM! Let's remove the workaround later in case you manage to get the change upstream.