Support bare worktrees where worktree does not have its own .git file

This was on oversight on my part: I assumed that the --work-tree arg was
always intended for use with linked worktrees which have a .git file
pointing back to the repo.

I'm honestly confused now: seems like there are three kinds of worktrees:
* the main worktree of a non-bare repo
* a linked worktree (with its own gitdir in the repo's worktrees/ dir)
* a random folder which you specify as a worktree with the --work-tree arg

I'm pretty sure the --work-tree arg is only intended to be used with this
third kind or workree

pkg/env/env.go

@@ -14,6 +14,15 @@ func SetGitDirEnv(value string) {
 	os.Setenv("GIT_DIR", value)
 }
 
-func UnsetGitDirEnv() {
-	_ = os.Unsetenv("GIT_DIR")
+func GetWorkTreeEnv() string {
+	return os.Getenv("GIT_WORK_TREE")
+}
+
+func SetWorkTreeEnv(value string) {
+	os.Setenv("GIT_WORK_TREE", value)
+}
+
+func UnsetGitLocationEnvVars() {
+	_ = os.Unsetenv("GIT_DIR")
+	_ = os.Unsetenv("GIT_WORK_TREE")
 }