filter-branch: fix ref rewriting with --subdirectory-filter

The previous ancestor discovery code failed on any refs that are
(pre-rewrite) ancestors of commits marked for rewriting.  This means
that in a situation

   A -- B(topic) -- C(master)

where B is dropped by --subdirectory-filter pruning, the 'topic' was
not moved up to A as intended, but left unrewritten because we asked
about 'git rev-list ^master topic', which does not return anything.

Instead, we use the straightforward

   git rev-list -1 $ref -- $filter_subdir

to find the right ancestor.  To justify this, note that the nearest
ancestor is unique: We use the output of

  git rev-list --parents -- $filter_subdir

to rewrite commits in the first pass, before any ref rewriting.  If B
is a non-merge commit, the only candidate is its parent.  If it is a
merge, there are two cases:

- All sides of the merge bring the same subdirectory contents.  Then
  rev-list already pruned away the merge in favour for just one of its
  parents, so there is only one candidate.

- Some merge sides, or the merge outcome, differ.  Then the merge is
  not pruned and can be rewritten directly.

So it is always safe to use rev-list -1.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -317,24 +317,19 @@ done <../revs
 
 # In case of a subdirectory filter, it is possible that a specified head
 # is not in the set of rewritten commits, because it was pruned by the
-# revision walker.  Fix it by mapping these heads to the next rewritten
-# ancestor(s), i.e. the boundaries in the set of rewritten commits.
+# revision walker.  Fix it by mapping these heads to the unique nearest
+# ancestor that survived the pruning.
 
-# NEEDSWORK: we should sort the unmapped refs topologically first
+if test "$filter_subdir"
+then
 	while read ref
 	do
 		sha1=$(git rev-parse "$ref"^0)
 		test -f "$workdir"/../map/$sha1 && continue
-	# Assign the boundarie(s) in the set of rewritten commits
-	# as the replacement commit(s).
-	# (This would look a bit nicer if --not --stdin worked.)
-	for p in $( (cd "$workdir"/../map; ls | sed "s/^/^/") |
-		git rev-list $ref --boundary --stdin |
-		sed -n "s/^-//p")
-	do
-		map $p >> "$workdir"/../map/$sha1
-	done
+		ancestor=$(git rev-list -1 $ref -- "$filter_subdir")
+		test "$ancestor" && echo $(map $ancestor) >> "$workdir"/../map/$sha1
 	done < "$tempdir"/heads
+fi
 
 # Finally update the refs
 

t/t7003-filter-branch.sh

@@ -101,7 +101,7 @@ test_expect_success 'filter subdirectory only' '
 		refs/heads/sub refs/heads/sub-earlier
 '
 
-test_expect_failure 'subdirectory filter result looks okay' '
+test_expect_success 'subdirectory filter result looks okay' '
 	test 2 = $(git rev-list sub | wc -l) &&
 	git show sub:new &&
 	test_must_fail git show sub:subdir &&