fast-import: avoid making replace refs point to themselves

fast-import: avoid making replace refs point to themselves

If someone replaces a commit with a modified version, then builds on
that commit, and then later decides to rewrite history in a format like

    git fast-export --all | CMD_TO_TWEAK_THE_STREAM | git fast-import

and CMD_TO_TWEAK_THE_STREAM undoes the modifications that the
replacement did, then at the end you'd get a replace ref that points to
itself.  For example:

    $ git show-ref | grep replace
    fb92ebc654641b310e7d0360d0a5a49316fd7264 refs/replace/fb92ebc654641b310e7d0360d0a5a49316fd7264

Git commands which pay attention to replace refs will die with an error
when a self-referencing replace ref is present:

    $ git log
    fatal: replace depth too high for object fb92ebc654641b310e7d0360d0a5a49316fd7264

Avoid such problems by deleting replace refs that will simply end up
pointing to themselves at the end of our writing.  Unless users specify
--quiet, warn them when we delete such a replace ref.

Two notes about this patch:
  * We are not ignoring the problematic update of the replace ref
    (turning it into a no-op), we are replacing the update with a delete.
    The logic here is that if the repository had a value for the replace
    ref before fast-import was run, and the replace ref was explicitly
    named in the fast-import stream, we don't want the replace ref to be
    left with a pre-fast-import value.
  * While loops with more than one element (e.g. refs/replace/A points
    to B, and refs/replace/B points to A) are possible, they seem much
    less plausible.  It is pretty easy to create a sequence of
    git-filter-repo commands that will trigger a self-referencing replace
    ref, but I do not know how to trigger a scenario with a cycle length
    greater than 1.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>