worktree: factor out dwim_branch function

worktree: factor out dwim_branch function

Factor out a dwim_branch function, which takes care of the dwim'ery in
'git worktree add <path>'.  It's not too much code currently, but we're
adding a new kind of dwim in a subsequent patch, at which point it makes
more sense to have it as a separate function.

Factor it out now to reduce the patch noise in the next patch.

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