completion: add completer for rev-parse

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
completion: prefer to use local git-completion.bash

git-completion.zsh looks in various "default" locations for
git-completion.bash.  During development, the location

  $(dirname ${funcsourcetrace[1]%:*})/git-completion.bash

is the most obvious and up-to-date version.  Push it up on the list of
locations.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>