t9700: make hash size independent

t9700: make hash size independent

The Perl test script for t9700 was matching on exactly 40 hex
characters.  With SHA-256, we'll have 64 hex-character object IDs.
Create a variable with a regex which matches exactly 40 or 64 hex
characters and use that to match the output.  Note that both of the uses
of this can be anchored, which makes the code simpler, so do that as
well.

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