Fix strip_path and improved output messages
cvsimport: strip all inappropriate tag strings

Certain characters such as "?" can be present in a CVS tag name, but
git does not allow these characters in tags. If git-cvsimport
encounters a CVS tag that git cannot handle, cvsimport will error and
refuse to continue the import beyond that point.

When importing CVS tags, strip all the inappropriate strings from the
tag names as we translate them to git tag names.

Provide more debugging information to the user if we've altered the
tag and the "git tag" command still fails. Also, warn the user if we
end up skipping an (unusable) tag altogether.

Signed-off-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>