Fix coding style for pointer

Code cleanup to use `git_buf`s instead of simple c strings.

Make some syntax change to follow coding style.

`git_buf_sanitize` is called with user-input, and wants to sanity-check
that input.  Allow it to return a value if the input was malformed in a
way that we cannot cope.

When calling `git_note_next`, we end up calling `git_iterator_advance`
but ignore its error code. The intent is that we do not want to return
an error if it returns `GIT_ITEROVER`, as we want to return that value
on the next invocation of `git_note_next`. We should still check for any
other error codes returned by `git_iterator_advance` to catch unexpected
internal errors.

Fix this by checking the function's return value, ignoring
`GIT_ITEROVER`.

Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the blob functions for
consistency with the rest of the library.

Our blob size is a `git_off_t`, which is a signed 64 bit int.  This may
be erroneously negative or larger than `SIZE_MAX`.  Ensure that the blob
size fits into a `size_t` before casting.

Move to the `git_error` name in the internal API for error-related
functions.