unquote_c_style: fix off-by-one.

unquote_c_style: fix off-by-one.

The optional endp parameter to unquote_c_style() was supposed to point at
a location past the closing double quote, but it was going one beyond it.

git-fast-import used this function heavily and the bug caused it to
misparse the input stream, especially when parsing a rename command:

	R "filename that needs quoting" rename-target-name

Because the function erroneously ate the whitespace after the closing dq,
this triggered "Missing space after source" error when it shouldn't.

Thanks to Adeodato Simò for having caught this.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>