This adds functionality to suggest last used branches (and commits) when
tab completing "git checkout" command. This is inspired by "@{-N}" syntax that
you can use use now in git (for example "git checkout @{-1}" switch to last used
branch.

We got this message from git when we ran git log command:
"fatal: ambiguous argument 'moving': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'"

This change in quoting fixes it.