git-p4: fix syncing file types with pattern

Example of pattern file type: text+k

Text filtered through the p4 pattern regexp must be converted from
string back to bytes, otherwise 'data' command for the fast-import
will receive extra invalid characters, followed by the fast-import
process error.

CC: Yang Zhao <yang.zhao@skyboxlabs.com>
Signed-off-by: Daniel Levin <dendy.ua@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
