When `GIT_OPT_ENABLE_STRICT_OBJECT_CREATION` is turned on, validate
the tree and parent ids given to commit creation functions.

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.

When we moved the logic to handle the first one, wrong loop logic was
kept in place which meant we still finished early. But we now notice it
because we're not reading past the last LF we find.

This was not noticed before as the last field in the tested commit was
multi-line which does not trigger the early break.

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

We were searching only past the first header field, which meant we were
unable to find e.g. `tree` which is the first field.

While here, make sure to set an error message in case we cannot find the
field.

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.

The current version of the commit creation and amend function are unsafe
to use when passing the update_ref parameter, as they do not check that
the reference at the moment of update points to what the user expects.

Make sure that we're moving history forward when we ask the library to
update the reference for us by checking that the first parent of the new
commit is the current value of the reference. We also make sure that the
ref we're updating hasn't moved between the read and the write.

Similarly, when amending a commit, make sure that the current tip of the
branch is the commit we're amending.

Any well-behaved program should write a descriptive message to the
reflog whenever it updates a reference. Let's make this more prominent
by removing the version without the reflog parameters.