cleanup duplicate name_compare() functions

cleanup duplicate name_compare() functions

We often represent our strings as a counted string, i.e. a pair of
the pointer to the beginning of the string and its length, and the
string may not be NUL terminated to that length.

To compare a pair of such counted strings, unpack-trees.c and
read-cache.c implement their own name_compare() functions
identically.  In addition, the cache_name_compare() function in
read-cache.c is nearly identical.  The only difference is when one
string is the prefix of the other string, in which case
name_compare() returns -1/+1 to show which one is longer, and
cache_name_compare() returns the difference of the lengths to show
the same information.

Unify these three functions by using the implementation from
cache_name_compare().  This does not make any difference to the
existing and future callers, as they must be paying attention only
to the sign of the returned value (and not the magnitude) because
the original implementations of these two functions return values
returned by memcmp(3) when the one string is not a prefix of the
other string, and the only thing memcmp(3) guarantees its callers is
the sign of the returned value, not the magnitude.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>