cvsimport: ease migration from CVSROOT/users format

cvsimport: ease migration from CVSROOT/users format

This fixes a minor bug, which caused the author email to be
doubly enclosed in a <> pair (the code gave enclosing <> to
GIT_AUTHOR_EMAIL and GIT_COMMITTER_EMAIL environment variable).

The read_author_info() subroutine is taught to also understand
the user list in CVSROOT/users format.  This is primarily done
to ease migration for CVS users, who can use the -A option
to read from existing CVSROOT/users file.  write_author_info()
always writes in the git-cvsimport's native format ('='
delimited and value without quotes).

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