completion: Support the DWIM mode for git checkout

completion: Support the DWIM mode for git checkout

Ever since commit 70c9ac2 (DWIM: "git checkout frotz" to "git checkout
-b frotz origin/frotz"), git checkout has supported a DWIM mode where
it creates a local tracking branch for a remote branch if just the name
of the remote branch is specified on the command-line and only one remote
has a branch with that name. Teach the bash completion script to understand
this DWIM mode and provide such remote-tracking branch names as possible
completions.

Signed-off-by: Kevin Ballard <kevin@sb.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>