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>

Documentation/git-cvsimport.txt

@@ -89,6 +89,26 @@ If you need to pass multiple options, separate them with a comma.
 -s <subst>::
 	Substitute the character "/" in branch names with <subst>
 
+-A <author-conv-file>::
+	CVS by default uses the unix username when writing its
+	commit logs. Using this option and an author-conv-file
+	in this format
+
+	exon=Andreas Ericsson <ae@op5.se>
+	spawn=Simon Pawn <spawn@frog-pond.org>
+
+	git-cvsimport will make it appear as those authors had
+	their GIT_AUTHOR_NAME and GIT_AUTHOR_EMAIL set properly
+	all along.
+
+	For convenience, this data is saved to $GIT_DIR/cvs-authors
+	each time the -A option is provided and read from that same
+	file each time git-cvsimport is run.
+
+	It is not recommended to use this feature if you intend to
+	export changes back to CVS again later with
+	git-link[1]::git-cvsexportcommit.
+
 OUTPUT
 ------
 If '-v' is specified, the script reports what it is doing.