And decrease extra reload checks of config data.

There are a few places where we need to join three strings to
assemble a path.  This adds a simple join3 function to avoid the
comparatively expensive join_n (which calls strlen on each string
twice).

On bare by default, or when core.logallrefupdates is false, we must not
write the reflog.

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.

Remove some duplicated logic.

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.