Clarify and correct -z

Clarify and correct -z

The description for -z is too vague and general for the
apply, diff*, and log commands.

Change the description of -z for 'git log' to note that
commits will be separated by NULs.

Change the description of -z for 'git diff*' and 'git apply'
to note that it applies to the --numstat option, and for
'git diff*' also for --raw option.

Also correct the description of the "munging" of pathanmes that
takes place in the absence of -z for the 'git diff*' and
'git apply' commands, namely that apart from the characters mentioned,
double quotes will also be escaped and that the pathname will be
enclosed in double quotes if any characters are escaped.

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>