worktree: prepare for more checks of whether path can become worktree

worktree: prepare for more checks of whether path can become worktree

Certain conditions must be met for a path to be a valid candidate as the
location of a new worktree; for instance, the path must not exist or
must be an empty directory. Although the number of conditions is small,
new conditions will soon be added so factor out the existing checks into
a separate function to avoid further bloating add_worktree().

Signed-off-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>