Ensure that the repository directory is owned by the current user; this
prevents us from opening configuration files that may have been created
by an attacker.

The more generic GIT_ERROR_SHA allows for SHA256 errors as well as SHA1.

The `git_error_set` function is useful for callers who implement
backends and advanced callbacks.  Expose it.

Provide macros to replace usages of `assert`.  A true `assert` is
punishing as a library.  Instead we should do our best to not crash.

GIT_ASSERT_ARG(x) will now assert that the given argument complies to
some format and sets an error message and returns `-1` if it does not.

GIT_ASSERT(x) is for internal usage, and available as an internal
consistency check.  It will set an error message and return `-1` in the
event of failure.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Disambiguate between general network problems and HTTP problems in error
codes.