range-diff: avoid negative string precision

range-diff: avoid negative string precision

If the supplied integer for "precision" is negative in
`"%.*s", len, line` then it is ignored. So the current code is
equivalent to just `"%s", line` because it is executed only if
`len` is negative.

Fix this by saving the value of `len` before overwriting it with the
return value of `parse_git_diff_header()`.

Signed-off-by: Vasil Dimov <vd@FreeBSD.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>