Rework unquote_c_style to work on a strbuf.

Rework unquote_c_style to work on a strbuf.

If the gain is not obvious in the diffstat, the resulting code is more
readable, _and_ in checkout-index/update-index we now reuse the same buffer
to unquote strings instead of always freeing/mallocing.

This also is more coherent with the next patch that reworks quoting
functions.

The quoting function is also made more efficient scanning for backslashes
and treating portions of strings without a backslash at once.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>