range-diff: drop useless "offset" variable from read_patches()

range-diff: drop useless "offset" variable from read_patches()

The "offset" variable was was introduced in 44b67cb62b (range-diff:
split lines manually, 2019-07-11), but it has never done anything
useful. We use it to count up the number of bytes we've consumed, but we
never look at the result. It was probably copied accidentally from an
almost-identical loop in apply.c:find_header() (and the point of that
commit was to make use of the parse_git_diff_header() function which
underlies both).

Because the variable was set but not used, most compilers didn't seem to
notice, but the upcoming clang-14 does complain about it, via its
-Wunused-but-set-variable warning.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>