git-cvsimport: Add -A <author-conv-file> option

git-cvsimport: Add -A <author-conv-file> option

This patch adds the option to specify an author name/email conversion
file in the format

	exon=Andreas Ericsson <ae@op5.se>
	spawn=Simon Pawn <spawn@frog-pond.org>

which will translate the ugly cvs authornames to the more informative
git style.

The info is saved in $GIT_DIR/cvs-authors, so that subsequent
incremental imports will use the same author-info even if no -A
option is specified. If an -A option *is* specified, the info in
$GIT_DIR/cvs-authors is appended/updated appropriately.

Docs updated accordingly.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>