t: switch $_x40 to $OID_REGEX

t: switch $_x40 to $OID_REGEX

Switch all uses of $_x40 to $OID_REGEX so that they work correctly with
larger hashes.  This commit was created by using the following sed
command to modify all files in the t directory except t/test-lib.sh:

  sed -i 's/\$_x40/$OID_REGEX/g'

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