preserving completion for git aliases.

git-aliases.sh

# define some aliases

alias m="git checkout master"
alias mp="git checkout master && git pull"
alias ci="git commit"
alias c="git checkout"
alias co="git checkout"
alias b="git checkout -b"
alias pr="git push mine && hub pull-request"
alias uncommit="git log -1 && git reset HEAD^1"

# register existing git completions for these aliases if git-completion is available

if [ ! -z "$(declare -f __git_complete)" ]; then
    __git_complete ci _git_commit
    __git_complete c _git_checkout
    __git_complete co _git_checkout
    __git_complete mp _git_pull
fi

For @ivanov, from SO