Define GIT_(AUTHOR|COMMITTER)_(NAME|EMAIL) to known values.

Without these, running tests with an account with empty gecos
field would fail.

We might want to loosen error from "git-var -l" (but not
"git-var GIT_AUTHOR_NAME") later, but that is more or less an
independent issue.

Signed-off-by: Junio C Hamano <junkio@cox.net>