t/test-lib: introduce OID_REGEX

t/test-lib: introduce OID_REGEX

Currently we have a variable, $_x40, which contains a regex that matches
a full 40-character hex constant.  However, with NewHash, we'll have
object IDs that are longer than 40 characters.  In such a case, $_x40
will be a confusing name.  Create a $OID_REGEX variable which will
always reflect a regex matching the appropriate object ID, regardless of
the length of the current hash.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>