How do you commit code as a different user?

Check out the --author option for git commit:

From the man page:

--author=<author>

Override the commit author. Specify an explicit author
using the standard A U Thor <author@example.com> format. Otherwise
<author> is assumed to be a pattern
and is used to search for an existing
commit by that author (i.e. rev-list --all -i --author=<author>); the commit author is then copied from the
first such commit found.