Use the new function to quote characters as they are being added to
buf, rather than quoting them in *p and then copying them into buf.
This increases code sharing, and changes the algorithm from O(N^2) to
O(N) in the number of characters in a line.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>