lstat_cache(): small cleanup and optimisation

lstat_cache(): small cleanup and optimisation

Simplify the if-else test in longest_match_lstat_cache() such that we
only have one simple if test.  Instead of testing for 'i == cache.len'
or 'i == len', we transform this to a common test for 'i == max_len'.

And to further optimise we use 'i >= max_len' instead of 'i ==
max_len', the reason is that it is now the exact opposite of one part
inside the while-loop termination expression 'i < max_len && name[i]
== cache.path[i]', and then the compiler can probably reuse a test
instruction from it.

We also throw away the arguments to reset_lstat_cache(), such that all
the safeguard logic inside lstat_cache() is handled at one place.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>