git-fast-import.txt: improve documentation for quoted paths

git-fast-import.txt: improve documentation for quoted paths

The documentation mentioned only newlines and double quotes as
characters needing escaping, but the backslash also needs it. Also, the
documentation was not clearly saying that double quotes around the file
name were required (double quotes in the examples could be interpreted as
part of the sentence, not part of the actual string).

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>