filter-branch: drop multiple-ancestor warning

filter-branch: drop multiple-ancestor warning

When a ref maps to a commit that is neither rewritten nor kept by
filter-branch (e.g., because it was eliminated by rev-list's pathspec
selection), we rewrite it to its nearest ancestor.

Since the initial commit in 6f6826c52b (Add git-filter-branch,
2007-06-03), we have warned when there are multiple such ancestors in
the map file. However, the warning code is impossible to trigger these
days. Since a0e46390d3 (filter-branch: fix ref rewriting with
--subdirectory-filter, 2008-08-12), we find the ancestor using "rev-list
-1", so it can only ever have a single value.

This code is made doubly confusing by the fact that we append to the map
file when mapping ancestors. However, this can never yield multiple
values because:

  - we explicitly check whether the map already exists, and if so, do
    nothing (so our "append" will always be to a file that does not
    exist)

  - even if we were to try mapping twice, the process to do so is
    deterministic. I.e., we'd always end up with the same ancestor for a
    given sha1. So warning about it would be pointless; there is no
    ambiguity.

So swap out the warning code for a BUG (which we'll simplify further in
the next commit). And let's stop using the append operator to make the
ancestor-mapping code less confusing.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>