There are a number of tests that modify the global or system
search paths during the tests.  This adds a helper function to
make it easier to restore those paths and makes sure that they
are getting restored in a manner that preserves test isolation.

Given HEAD -> master -> foo, when updating foo's reflog we should also
update HEAD's, as it's considered the current branch.

The reflog append function was overzealous in its checking. When passed
an old and new ids, it should not do any checking, but just serialize
the data to a reflog entry.

If the caller wants to update a ref to point to the same target as it
currently has, we should return early and avoid writing to the reflog.

The existing ones lack checking zeroed ids when switching back from an
unborn branch as well as what happens when detaching.

The reflog appending function mistakenly wrote zeros when dealing with a
detached HEAD. This explicitly checks for those situations and fixes
them.

When we update the current branch, we must also append to HEAD's reflog
to keep them in sync.

This is a bit of a hack, but as git.git says, it covers 100% of
default cases.