Use the new object_type enumeration names within the codebase.

Use the new-style index names throughout our own codebase.

C++ style comment ("//") are not specified by the ISO C90 standard and
thus do not conform to it. While libgit2 aims to conform to C90, we did
not enforce it until now, which is why quite a lot of these
non-conforming comments have snuck into our codebase. Do a tree-wide
conversion of all C++ style comments to the supported C style comments
to allow us enforcing strict C90 compliance in a later commit.

Now that the index has a "dirty" state, where it has changes that have
not yet been committed or rolled back, our tests need to be adapted to
actually commit or rollback the changes instead of assuming that the
index can be operated on in its indeterminate state.

The index::reuc tests must test that the checkout itself succeeds,
otherwise subsequent tests are not valid.

In fact, the checkouts were failing because when checking out `SAFE`,
they cannot update the files that are in conflict.  Change the checkout
level to `FORCE` to ensure that they get updated correctly.

This function is meant to simulate what git does in the reset command,
so we should include the reflog message in that.

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.

Signed-off-by: Sven Strickroth <email@cs-ware.de>