From mboxrd@z Thu Jan  1 00:00:00 1970
From: Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH 2/2] filter-branch: subdirectory filter needs --full-history
Date: Tue, 12 Jun 2007 23:58:53 -0700
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Cc: Johannes Sixt <johannes.sixt@telecom.at>, git@vger.kernel.org,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
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In-Reply-To: <alpine.LFD.0.98.0706081822500.4205@woody.linux-foundation.org>
	(Linus Torvalds's message of "Fri, 8 Jun 2007 19:40:46 -0700 (PDT)")
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Linus Torvalds <torvalds@linux-foundation.org> writes:

> ... There was no actual developmet on branch 
> "another", so all the commits went away, but it left the merge (because 
> that's how --full-history works), which has now become pointless.
>
> So you should do a "merge cleanup" phase after running --full-history.

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)

However, I suspect that this is impossible without making
--full-history with "merge clean-up" imply "limited".

In the case of filter-branch, it needs to use topo-order in
order to guarantee that all its parents commits have been
rewritten before rewriting a commit, so the traversal is already
limited.  

It might be simpler for filter-branch to remove parents that are
ancestor of other parents ("git show-branch --independent" would
work well for this, I think), before feeding the list of mapped
parents to the parent-filter.