Create `git_email_*` which will encapsulate email creation and
application, and `git_email_create_from_commit` in particular, which
creates an email for a single commit.

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.

Introduce `git_apply_tree`, which will apply a `git_diff` to a given
`git_tree`, allowing an in-memory patch application for a repository.

I'm not sure if worktree.h was intentionally left out of git2.h. Looks like an oversight since it is in fact documented.

It is currently unused; it will go into the remote's options.

This was forgotten when the feature was implemented.