Hi Jeff,

After reading your reponse and re-reading my original email, I
realised it was totally unclear
so I have re-explained myself below.

2008/5/9 Jeff King <peff@peff.net>:
quoted text
> On Fri, May 09, 2008 at 11:01:47AM +1000, James Sadler wrote:
>
>> I have a git repository that I wish to split into multiple separate
>> repositories for each logical module that it contains. Each logical
>> module is already in its own directory at the root of the repo.
>
> OK.
>
>> To extract a module into its own repo, I first copied the original
>> repo (this was a simple cp -r, as it seemed to be the simplest way as
>> git clone doesn't get all the branches)

I must have experienced a brain fart or something or missed the '-r' from
git branch...

quoted text
>> and ran filter-branch with a --commit-filter to skip commits that were
>> irrelevant to th subdir.
>
> But that's part of what subdirectory-filter does, so this step is
> unnecessary.

Yes that's true, but...

Clearer explanation:

I originally tried --subdirectory-filter by itself to see if it would
do the job, but it filtered
more commits than I thought it should (some commits that touched the subdir were
missing after filter-branch was run).

I then began to question my understanding of the semantics of
subdirectory-filter.

Is it meant to:
A) Only keep commits where ALL of the changes in the commit only touch
content under $DIR?
B) Only keep commits where SOME of the changes in the commit touch
content under $DIR?

I suspected that it was behaving as A.

That's when I decided to run the commit-filter first in combination
with the tree-filter.  This would
leave me with all commits that touched the subdir but any commit that
touched multiple subdirs
would be cleaned up so it only touched the subdir I want to keep.

At this point I have a bunch of commits that only make changes to
subdir (verified using gitk), and I would
expect subdirectory-filter to keep every single commit.

However, after running it, I loose most of my commits.  Strangely, the
working tree is bit-for-bit correct
with the original version or the subdir in the old repo, but the
history leading up to it is not.

--subdirectory-filter does not seem to behave as either A or B above
but something other way.  I'm sure
it will turn out to be something silly, but I'm pulling my hair out
trying to figure this one out.

Hopefully that's a clearer explanation!

-- 
James
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