perl: create and switch variables for hash constants

git-svn has several variables for SHA-1 constants, including short hash
values and full length hash values.  Since these are no longer SHA-1
specific, let's start them with "oid" instead of "sha1".  Add a
constant, oid_length, which is the length of the hash algorithm in use
in hex.  We use the hex version because overwhelmingly that's what's
used by git-svn.

We don't currently set oid_length based on the repository algorithm, but
we will in a future commit.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Acked-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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