common_prefix: simplify and fix scanning for prefixes

common_prefix: simplify and fix scanning for prefixes

common_prefix() scans backwards from the far end of each 'next'
pathspec, starting from 'len', shortening the 'prefix' using 'path' as
a reference.

However, there is a small opportunity for an out-of-bounds access
because len is unconditionally set to prefix-1 after a "direct match"
test failed.  This means that if 'next' is shorter than prefix+2, we
read past it.

Instead of a minimal fix, simplify the loop: scan *forward* over the
'next' entry, remembering the last '/' where it matched the prefix
known so far.  This is far easier to read and also has the advantage
that we only scan over each entry once.

Acked-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>