Add git_status_file_at

We default to off, but we might want to consider changing `GIT_DIFF_NORMAL` to
include it.

Git considers '-0000' a valid offset for signature lines.  They need to
be treated as _not_ equal to a '+0000' signature offset.  Parsing a
signature line stores the offset in a signed integer which does not
distinguish between `+0` and `-0`.

This patch adds an additional flag `sign` to the `git_time` in the
`signature` object which is populated with the sign of the offset.  In
addition to exposing this information to the user, this information is
also used to compare signatures.

/cc @pks-t @ethomson

optios -> options

Our current configuration logic is completely oblivious of any
repository, but only cares for actual file paths. Unfortunately, we are
forced to break this assumption by the introduction of conditional
includes, which are evaluated in the context of a repository. Right now,
only one conditional exists with "gitdir:" -- it will only include the
configuration if the current repository's git directory matches the
value passed to "gitdir:".

To support these conditionals, we have to break our API and make the
repository available when opening a configuration file. This commit
extends the `open` call of configuration backends to include another
repository and adjusts existing code to have it available. This includes
the user-visible functions `git_config_add_file_ondisk` and
`git_config_add_backend`.

Several functions to retrieve variables from a repository only return
immutable values, which allows us to actually constify the passed-in
repository parameter. Do so to help a later patch, which will only have
access to a constant repository.

Very many callbacks in libgit2 have some sort of typedef to normalize the name at git_<name_of_operation>_cb. Add a typedef for push_update_references in the remote so the name follows the same conventions.