Introduce `git_oid_from_string`, `git_oid_from_prefix`, and
`git_oid_from_raw`, all of which take a `git_oid_t` that indicates what
type of OID should be parsed (SHA1 or SHA256).
This allows users to continue to use `git_oid_fromstr` without any code
changes, while remaining in a SHA1 world.
Note that these are not perfect analogs to the `fromstr` APIs.
* `git_oid_from_string` now takes a NUL terminated string, instead of
allowing for non-NUL terminated strings. Adding a NUL check feels like
an important safety consideration for C strings.
* `git_oid_from_prefix` should be used for an OID substring and length.
Previous usages of `git_oid_fromstr` with non-NUL terminated strings
should move to `git_oid_from_prefix` with the hexsize for the given OID
type.
c26d8a8b54