builtin_diff(): read $GIT_DIFF_OPTS closer to use

builtin_diff(): read $GIT_DIFF_OPTS closer to use

The value returned by getenv() is not guaranteed to remain valid across
other environment function calls. But in between our call and using the
value, we run fill_textconv(), which may do quite a bit of work,
including spawning sub-processes.

We can make this safer by calling getenv() right before we actually look
at its value.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>