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`.

whitespace.  Collapse spaces around newlines for the summary.

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.

Some tools create multiple author fields. git is rather lax when parsing
them, although fsck does complain about them. This means that they exist
in the wild.

As it's not too taxing to check for them, and there shouldn't be a
noticeable slowdown when dealing with correct commits, add logic to skip
over these extra fields when parsing the commit.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.

A signature is made up of a non-empty name and a non-empty email so
let's validate that. This also brings us more in line with git, which
also rejects ident with an empty email.