Assorted Coverity fixes

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.

The path given to `git_index_add_bypath` is relative to the root of the
repository. That `describe/file` path is relative to the root of the
sandbox directory, hence if I add the missing `cl_git_pass` I rightfully
get an error that `$SANDBOX/describe/describe/file doesn't exist`.

The path is thus changed to be made relative to the repository, which
makes the failure go away and "restore" the test.

Reported by Coverity, CID 1393678-1393697.

When no reference names could be found we did error out when trying to describe
a commit. This is wrong, though, when the option to fall back to a commit's
object ID is set.

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.

If there is a failure then cl_git_pass tries to get the libgit2
error, but p_... functions don't set that.

Also - trailing whitespace cleanup.

And implement the option init functions for this and the format options.

This makes us be closer to git's tests, and lets us better describe what
we expect from the output.