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 <t.gummerer@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>