* feat: Watch $GIT_DIR for git changes, if set
While rarely used, it's possible to set the $GIT_DIR environment
variable to instruct git to use a directory other than `.git`.
This checks if that environment variable is set; if it is, the plugin
will watch that directory. If it's not set, it'll fall back to the
default `.git` directory.
* fix: Don't create two watchers for $GIT_DIR
This will ignore a path for watching if EITHER it's '.git', or the value
of $GIT_DIR (if it's set).
If $GIT_DIR is not set, the vim.env object returns `nil`, which will
never match `path`.
* fix: Attempt to make a relative $GIT_DIR absolute
a6daf50b9d