Add core.quotepath configuration variable.

Add core.quotepath configuration variable.

We always quote "unusual" byte values in a pathname using
C-string style, to make it safer for parsing scripts that do not
handle NUL separated records well (or just too lazy to bother).
The absolute minimum bytes that need to be quoted for this
purpose are TAB, LF (and other control characters), double quote
and backslash.

However, we have also always quoted the bytes in high 8-bit
range; this was partly because we were lazy and partly because
we were being cautious.

This introduces an internal "quote_path_fully" variable, and
core.quotepath configuration variable to control it.  When set
to false, it does not quote bytes in high 8-bit range anymore
but passes them intact.

The variable defaults to "true" to retain the traditional
behaviour for now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>