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.

Fire progress and update tips callbacks also for pushes.

Improve test coverage of status with different core.precomposeunicode settings

Improve stash and checkout for ignored + untracked items

Merge checkout strategy

and make tests empty on platforms without iconv support.

There is an interesting difference with core Git here, though.
Because libgit2 will do rename detection with the working directory,
in the last case where the HEAD and the working directory both
have the decomposed data and the index has the composed data, we
generate a single status record with two renames whereas Git will
generate one rename (head to index) and one untracked file.

The current FETCH_HEAD parsing code assumes that a quote must end the
branch name. Git however allows for quotes as part of a branch name,
which causes us to consider the FETCH_HEAD file as invalid.

Instead of searching for a single quote char, search for a quote char
followed by SP, which is not a valid part of a ref name.