Re: combine git repo historically

[Jakub Narebski <jnareb@gmail.com>]

Christian Couder <chriscool@tuxfamily.org> writes:

> On Saturday 10 October 2009, bill lam wrote:

> > Christian Couder also mentioned the git-replace command, how to stitch
> > with it?
> 
> First you need to create a commit that will replace commit N, let's call it 
> N' and after that you can use "git replace N N'". So the complex part is to 
> create N'.
> 
> You want N' to have the same content as N but to have M as parent. So you 
> could do something like the following:
> 
> (We suppose that commits A to M are in branch1 and that you are in the root 
> directory of your repo2 working directory.)
> 
> $ git checkout -b repo1-branch1 remote/repo1/branch1
> $ git checkout N -- .
> $ export GIT_AUTHOR_NAME=<author name of commit N>
> $ export GIT_AUTHOR_EMAIL=<author email of commit N>
> $ export GIT_AUTHOR_DATE=<date of commit N>
> $ git commit -a
> 
> And then use the commit message from commit N, and maybe also say in the 
> commit message that it is replacement commit made to link repo1 with repo2 
> or something like that.

I think simpler solution would be to use plumbing for that.  First
save commit to be replaced in a (text) file:

 $ git cat-file commit N > COMMIT_N

If you want to edit only commit message and perhaps parentage (like I
think in this case), you need to simply edit COMMIT_N file, and modify
(or add) 'parent' header(s), which is between 'tree' and 'author'
header.

Then put modified commit in repository object database

 $ git hash-object -t commit -w COMMIT_N
 
> At this step you have created N' and you should make sure that it is exactly 
> what you want. It should point to the same tree as N, it should have M as 
> parent, ...
> 
> If everything is ok then you can use:
> 
> $ git replace N HEAD

And then do

 $ git replace N NEW_N

where N is _SHA-1_ of original comit ("git rev-parse --verify N^0"),
and NEW_N is SHA-1 of replacement commit, as written by git-hash-object.
 
> And you should be done.

But I haven't tested this. YMMV.

See also my answer here: 
http://stackoverflow.com/questions/1220557/how-do-i-prepend-history-to-a-git-repo/1547490#1547490

-- 
Jakub Narebski
Poland
ShadeHawk on #git