How to set up completion for git aliases?

[Vadim Zeitlin <vz-zsh@xxxxxxxxxxxx>]

 Hello,

 I have a simple git alias "lg = log --oneline" and I'd like zsh to
complete it in the same way as it already completes "git log".
Unfortunately after spending a long time looking at
/usr/share/zsh/functions/Completion/Unix/_git I still have no idea how am I
supposed to do it "properly", i.e. without copying this huge file and
hacking on it. I'm sure there must be a better way to do it but which one?

 Thanks in advance for any hints,
VZ

Attachment: pgp4kvM1RGUhS.pgp
Description: PGP signature