[PATCH] filter-branch: use --simplify-merges

[Thomas Rast <trast@student.ethz.ch>]

Use rev-list --simplify-merges everywhere.  This changes the behaviour
of --subdirectory-filter in cases such as

  O -- A -\
   \       \
    \- B -- M

where A and B bring the same changes to the subdirectory: It now keeps
both sides of the merge.  Previously, the history would have been
simplified to 'O -- A'.  Merges of unrelated side histories that never
touch the subdirectory are still removed.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
---

This obviously depends on --simplify-merges which is only in 'next'.

Junio C Hamano wrote:
>
> Perhaps --full-history is needed to the rev-list call (and the recent
> invention --simplify-merges that will hopefully appear sometime after
> 1.6.0)?  See recent discussion of --full-history and the default merge
> simplification between Linus and Roman Zippel.

Following history pointers, it turns out the discussion surrounding
a17171b4 (Revert "filter-branch: subdirectory filter needs
--full-history") actually mentions that a simplification step on top
of --full-history is needed:

Junio C Hamano wrote:
[http://kerneltrap.org/mailarchive/git/2007/6/13/249107]
> In short,
> you will end up with something like this:
> 
>              .---. (side branch)
>             /     \
>         ---A---B---C (merge)
> 
> The "merge clean-up" would conceptually be a simple operation.
> Whenever you see a merge C, you look at its parents A and B, and
> cull the ones that are reachable from other parents.  You notice
> that A is an ancestor of B, drop A from the parents of C, and
> simplify the above down to:
> 
>         ---A---B---C (not-a-merge)

Well, turns out that's what you did with --simplify-merges, so let's
use it.

 git-filter-branch.sh |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/git-filter-branch.sh b/git-filter-branch.sh
index 539b2e6..60f64ac 100755
--- a/git-filter-branch.sh
+++ b/git-filter-branch.sh
@@ -239,11 +239,11 @@ mkdir ../map || die "Could not create map/ directory"
 case "$filter_subdir" in
 "")
 	git rev-list --reverse --topo-order --default HEAD \
-		--parents "$@"
+		--parents --simplify-merges "$@"
 	;;
 *)
 	git rev-list --reverse --topo-order --default HEAD \
-		--parents "$@" -- "$filter_subdir"
+		--parents --simplify-merges "$@" -- "$filter_subdir"
 esac > ../revs || die "Could not get the commits"
 commits=$(wc -l <../revs | tr -d " ")
 
@@ -333,7 +333,8 @@ then
 	do
 		sha1=$(git rev-parse "$ref"^0)
 		test -f "$workdir"/../map/$sha1 && continue
-		ancestor=$(git rev-list -1 $ref -- "$filter_subdir")
+		ancestor=$(git rev-list --simplify-merges -1 \
+				$ref -- "$filter_subdir")
 		test "$ancestor" && echo $(map $ancestor) >> "$workdir"/../map/$sha1
 	done < "$tempdir"/heads
 fi
-- 
1.6.0.rc2.29.g7ec81