filter-branch: nearest-ancestor rewriting outside subdir filter

Since a0e4639 (filter-branch: fix ref rewriting with
--subdirectory-filter, 2008-08-12) git-filter-branch has done
nearest-ancestor rewriting when using a --subdirectory-filter.

However, that rewriting strategy is also a useful building block in
other tasks.  For example, if you want to split out a subset of files
from your history, you would typically call

  git filter-branch -- <refs> -- <files>

But this fails for all refs that do not point directly to a commit
that affects <files>, because their referenced commit will not be
rewritten and the ref remains untouched.

The code was already there for the --subdirectory-filter case, so just
introduce an option that enables it independently.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -159,7 +159,18 @@ to other tags will be rewritten to point to the underlying commit.
 --subdirectory-filter <directory>::
 	Only look at the history which touches the given subdirectory.
 	The result will contain that directory (and only that) as its
-	project root.
+	project root.  Implies --remap-to-ancestor.
+
+--remap-to-ancestor::
+	Rewrite refs to the nearest rewritten ancestor instead of
+	ignoring them.
++
+Normally, positive refs on the command line are only changed if the
+commit they point to was rewritten.  However, you can limit the extent
+of this rewriting by using linkgit:rev-list[1] arguments, e.g., path
+limiters.  Refs pointing to such excluded commits would then normally
+be ignored.  With this option, they are instead rewritten to point at
+the nearest ancestor that was not excluded.
 
 --prune-empty::
 	Some kind of filters will generate empty commits, that left the tree

git-filter-branch.sh

@@ -125,6 +125,7 @@ filter_subdir=
 orig_namespace=refs/original/
 force=
 prune_empty=
+remap_to_ancestor=
 while :
 do
 	case "$1" in
@@ -137,6 +138,11 @@ do
 		force=t
 		continue
 		;;
+	--remap-to-ancestor)
+		shift
+		remap_to_ancestor=t
+		continue
+		;;
 	--prune-empty)
 		shift
 		prune_empty=t
@@ -182,6 +188,7 @@ do
 		;;
 	--subdirectory-filter)
 		filter_subdir="$OPTARG"
+		remap_to_ancestor=t
 		;;
 	--original)
 		orig_namespace=$(expr "$OPTARG/" : '\(.*[^/]\)/*$')/
@@ -354,12 +361,13 @@ while read commit parents; do
 			die "could not write rewritten commit"
 done <../revs
 
-# In case of a subdirectory filter, it is possible that a specified head
+# If we are filtering for paths, as in the case of a subdirectory
-# is not in the set of rewritten commits, because it was pruned by the
+# filter, it is possible that a specified head is not in the set of
-# revision walker.  Fix it by mapping these heads to the unique nearest
+# rewritten commits, because it was pruned by the revision walker.
-# ancestor that survived the pruning.
+# Ancestor remapping fixes this by mapping these heads to the unique
+# nearest ancestor that survived the pruning.
 
-if test "$filter_subdir"
+if test "$remap_to_ancestor" = t
 then
 	while read ref
 	do

t/t7003-filter-branch.sh

@@ -288,4 +288,22 @@ test_expect_success 'Prune empty commits' '
 	test_cmp expect actual
 '
 
+test_expect_success '--remap-to-ancestor with filename filters' '
+	git checkout master &&
+	git reset --hard A &&
+	test_commit add-foo foo 1 &&
+	git branch moved-foo &&
+	test_commit add-bar bar a &&
+	git branch invariant &&
+	orig_invariant=$(git rev-parse invariant) &&
+	git branch moved-bar &&
+	test_commit change-foo foo 2 &&
+	git filter-branch -f --remap-to-ancestor \
+		moved-foo moved-bar A..master \
+		-- -- foo &&
+	test $(git rev-parse moved-foo) = $(git rev-parse moved-bar) &&
+	test $(git rev-parse moved-foo) = $(git rev-parse master^) &&
+	test $orig_invariant = $(git rev-parse invariant)
+'
+
 test_done