t7700: make references to SHA-1 generic

t7700: make references to SHA-1 generic

Make the test more hash-agnostic by renaming variables from "sha1" to
some variation of "oid" or "packid". Also, replace the regex,
`[0-9a-f]\{40\}` with `$OID_REGEX`.

A better name for "incrpackid" (incremental pack-id) might have been
just "packid". However, later in the test suite, we have other uses of
"packid". Although the scopes of these variables don't conflict, a
future developer may think that commit_and_pack() and
test_has_duplicate_object() are semantically related somehow since they
share the same variable name. Give them distinct names so that it's
clear these uses are unrelated.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>