Any well-behaved program should write a descriptive message to the
reflog whenever it updates a reference. Let's make this more prominent
by removing the version without the reflog parameters.

git-core only writes to the reflogs of HEAD, refs/heads/ and,
refs/notes/ or if there is already a reflog in place. Adjust our code to
follow these semantics.

Before these changes, looking up a reference would return the
same precomposed or decomposed form of the reference name that
was used to look it up, so on MacOS which ignores the difference
between the two, a single reference could be looked up either way
and git_reference_name would return the form of the name that was
used to look it up!  This change makes lookup always return the
precomposed name if core.precomposeunicode is set regardless of
which version was used to look it up.  The reference iterator was
already returning the precomposed form from earlier work.

This also updates the CMakeLists.txt rules for enabling iconv
usage because the clar tests for this code were actually not being
activated properly with the old version.

Finally, this moves git_repository_reset_filesystem from include/
git2/repository.h to include/git2/sys/repository.h since it is not
really a function that normal library users should have to think
about very often.

Thread safety for the refdb_fs

The refdb_fs implementation calls realloc directly on a reference
object when it wants to rename it.  It is not a public object, so
this doesn't mess with the immutability of references, but it does
assume certain constraints on the reference representation.  This
commit wraps that assumption in an isolated API to isolate it.

This makes parsing easier! :p