Git autocomplete for custom bash functions

The recommended method is by using __git_complete():

__git_complete gitpull _git_pull

Manually-crafted bash completion is as simple as this:

# our handler that returns choices by populating Bash array COMPREPLY
# (filtered by the currently entered word ($2) via compgen builtin)
_gitpull_complete() {
    branches=$(git branch -l | cut -c3-)
    COMPREPLY=($(compgen -W "$branches" -- "$2"))
}

# we now register our handler to provide completion hints for the "gitpull" command
complete -F _gitpull_complete gitpull

After sourcing the above commands:

$ gitpull <TAB>
asd     master  qwe     zxc
$ gitpull m<TAB>
$ gitpull master

The ultimate reference on bash completion is (of course) the section on Programmable Completion in the bash manual, but a nice introduction is given on "Debian Administration" page (part 1 and a more important part 2).