We should not be in the business of copying strings around for users.
We either return a strarray that can be freed, or we take one (and do
not mutate it).

We _dispose_ the contents of objects; we _free_ objects (and their
contents).  Update `git_strarray_free` to be `git_strarray_dispose`.
`git_strarray_free` remains as a deprecated proxy function.

The credential structures are now opaque and defined in
`sys/credential.h`.  However, we should continue to provide them for
backward compatibility, unless `GIT_DEPRECATED_HARD` is set.

We avoid abbreviations where possible; rename git_cred to
git_credential.

In addition, we have standardized on a trailing `_t` for enum types,
instead of using "type" in the name.  So `git_credtype_t` has become
`git_credential_t` and its members have become `GIT_CREDENTIAL` instead
of `GIT_CREDTYPE`.

Finally, the source and header files have been renamed to `credential`
instead of `cred`.

Keep previous name and values as deprecated, and include the new header
files from the previous ones.

Some libraries haven't updated to git_attr_value_t and break.  Adding
the comapt typedef as suggested.

Optionally read `.gitattributes` from HEAD

Users should now use `git_blob_filter`.

In #5118 we remove the double-underscore to make it a normally-named public
function. However, this is not an interesting function outside of the library
and it takes up a name for something that could be more useful.

Remove the single-underscore version as we have not done any releases with it.