Move to the `git_error` name in error-related functions, deprecating the
`giterr` functions.  This means, for example, that `giterr_last` is now
`git_error_last`.  The old names are retained for compatibility.

This only updates the public API; internal API and function usage
remains unchanged.

The functions `git_commit_header_field` and
`git_commit_extract_signature` both receive buffers used to hand back
the results to the user. While these functions called `git_buf_sanitize`
on these buffers, this is not the right thing to do, as it will simply
initialize or zero-terminate passed buffers. As we want to overwrite
contents, we instead have to call `git_buf_clear` to completely reset
them.

In combination with the function which creates a commit into a buffer,
this allows us to more easily create signed commits.

Sometimes you want to create a commit but not write it out to the
objectdb immediately. For these cases, provide a new function to
retrieve the buffer instead of having to go through the db.

We should be checking whether the object we're looking up is a commit,
and we should let the caller know whether the not-found return code
comes from a bad object type or just a missing signature.

This returns the GPG signature for a commit and its contents without the
signature block, allowing for the verification of the commit's
signature.

It is already possible to get a commit's summary with the
`git_commit_summary` function. It is not possible to get the
remaining part of the commit message, that is the commit
message's body.

Fix this by introducing a new function `git_commit_body`.

This allows the user to look up fields which we don't parse in libgit2,
and allows them to access gpgsig or mergetag fields if they wish to
check the signature.