Nul terminated oid

This PR is in response to issue #770.

I introduced a new oid function, `git_oid_fromstrp` that thats a null terminated string as argument.

Doing that, I noticed that the `git_oid_fromstrn` function does to check to ensure that the string is at least GIT_OID_MINPREFIXLEN characters long. Should I add it to this PR or create another one?