dir.c::match_basename(): pay attention to the length of string parameters

dir.c::match_basename(): pay attention to the length of string parameters

The function takes two counted strings (<basename, basenamelen> and
<pattern, patternlen>) as parameters, together with prefix (the
length of the prefix in pattern that is to be matched literally
without globbing against the basename) and EXC_* flags that tells it
how to match the pattern against the basename.

However, it did not pay attention to the length of these counted
strings.  Update them to do the following:

 * When the entire pattern is to be matched literally, the pattern
   matches the basename only when the lengths of them are the same,
   and they match up to that length.

 * When the pattern is "*" followed by a string to be matched
   literally, make sure that the basenamelen is equal or longer than
   the "literal" part of the pattern, and the tail of the basename
   string matches that literal part.

 * Otherwise, use the new fnmatch_icase_mem helper to make
   sure we only lookmake sure we use only look at the
   counted part of the strings.  Because these counted strings are
   full strings most of the time, we check for termination
   to avoid unnecessary allocation.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>