worktree: add --guess-remote flag to add subcommand

worktree: add --guess-remote flag to add subcommand

Currently 'git worktree add <path>' creates a new branch named after the
basename of the <path>, that matches the HEAD of whichever worktree we
were on when calling "git worktree add <path>".

It's sometimes useful to have 'git worktree add <path> behave more like
the dwim machinery in 'git checkout <new-branch>', i.e. check if the new
branch name, derived from the basename of the <path>, uniquely matches
the branch name of a remote-tracking branch, and if so check out that
branch and set the upstream to the remote-tracking branch.

Add a new --guess-remote option that enables exactly that behaviour.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>