fast-export: quote paths in output

fast-export: quote paths in output

Many pathnames in a fast-import stream need to be quoted. In
particular:

  1. Pathnames at the end of an "M" or "D" line need quoting
     if they contain a LF or start with double-quote.

  2. Pathnames on a "C" or "R" line need quoting as above,
     but also if they contain spaces.

For (1), we weren't quoting at all. For (2), we put
double-quotes around the paths to handle spaces, but ignored
the possibility that they would need further quoting.

This patch checks whether each pathname needs c-style
quoting, and uses it. This is slightly overkill for (1),
which doesn't actually need to quote many characters that
vanilla c-style quoting does. However, it shouldn't hurt, as
any implementation needs to be ready to handle quoted
strings anyway.

In addition to adding a test, we have to tweak a test which
blindly assumed that case (2) would always use
double-quotes, whether it needed to or not.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>