cvsimport: apply shell-quoting regex globally

Commit 5b4efea666 (cvsimport: shell-quote variable used in
backticks, 2017-09-11) tried to shell-quote a variable, but
forgot to use the "/g" modifier to apply the quoting to the
whole variable. This means we'd miss any embedded
single-quotes after the first one.

Reported-by: <littlelailo@yahoo.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
