[RFC PATCH] Simplify away duplicate commits with --cherry-pick --parents

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

The current --cherry-pick declares commits SHOWN that are found to be
duplicates.  Unfortunately this disconnects the history at every such
duplicate, making it quite hard to follow in graphical viewers.

Add an extra stage of parent rewriting after scanning for duplicates,
which simplifies the history to omit all duplicate commits.  This
cannot easily be shifted to the existing parent rewriting because
cherry_pick_list() always comes last in the entire filtering process
(presumably because it is the most expensive).

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

I wrote:
> The problem with [gitk --left-right --cherry-pick A...B] is that it
> disconnects history
[...]
> Sadly, it's really the underlying git-rev-list that is "broken" in the
> sense that it does not fix the parent lists.  And git log --graph
> handles it much worse than gitk.

Maybe this is an approach.  It unfortunately breaks down if merges can
disappear because of patch-ids too.  Can they?

(In the case where a merge is flagged SHOWN, it might have its parent
list reduced to one at some point, and then later filterings would
simplify it away whereas earlier ones didn't.)

Also, I'm not entirely sure we want to do this without any guards
except rewrite_parents.

On the plus side, the issues with git log --graph vanish because
history is again connected :-)


 revision.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/revision.c b/revision.c
index 9f5dac5..9e24514 100644
--- a/revision.c
+++ b/revision.c
@@ -517,6 +517,8 @@ static int add_parents_to_list(struct rev_info *revs, struct commit *commit,
 	return 0;
 }
 
+static int remove_duplicate_parents(struct commit *commit);
+
 static void cherry_pick_list(struct commit_list *list, struct rev_info *revs)
 {
 	struct commit_list *p;
@@ -599,6 +601,25 @@ static void cherry_pick_list(struct commit_list *list, struct rev_info *revs)
 		commit->util = NULL;
 	}
 
+	if (revs->rewrite_parents) {
+		/* Prune away commits we've just found to be duplicates */
+		for (p = list; p; p = p->next) {
+			struct commit *commit = p->item;
+			struct commit_list *pp;
+
+			for (pp = commit->parents; pp; pp = pp->next) {
+				struct commit *parent = pp->item;
+				while (parent->object.flags & SHOWN
+				       && parent->parents
+				       && !parent->parents->next)
+					parent = parent->parents->item;
+				pp->item = parent;
+			}
+
+			remove_duplicate_parents(commit);
+		}
+	}
+
 	free_patch_ids(&ids);
 }
 
-- 
1.6.4.199.g24c3