Subject: Re: Merge two different repositories (v2.4 + v2.5) into the one (v2.4 -> v2.5). Possible?

From: Jakub Narebski <[email-masked]>
Date: Tue, 11 Jan 2011 05:21:48 -0800 (PST)

=?windows-1251?B?wOvl6vHl6SDY8+zq6O0=?= <[email-masked]> writes:

> 1.5 years ago I had sources of a project in a SVN repository (actually it does not
> matter what SCM was used before). And I had two branches: v2.4 and v2.5.
> They differed enough at that moment and (as usual for SVN branches)
> laid in two different folders.
> Then I had known of Git and I decided to try to use this powerful DVCS.
> But as I was a newbie I created two git-repositories: one per each
> branch. So v2.4 has its own git-repo. v2.5 (and above) has another one.
> 
> Now I'd like to merge them as v2.5 was a continuos branch from v2.4,
> but without a rebasing (i.e. without a global changing of v2.5
> repository, which already has another branches)
> It must look like LAST commit of v2.4 should be a PARENT of FIRST commit of v2.5
> 
> Now there's a question: Is it possible to do so (no rebasing!), and If
> "yes" then how to?

As Andreas Ericsson wrote, you can do this using grafts (and you can
make history with grafts permanent using "git filter-branch").

Better solution might be to use more modern replace mechanism, see
e.g. "git replace" manpage.  Below there is untested step-by-step
instruction.

First, you have put history of v2.4 and of v2.5 in a single repository
(e.g. using "git remote add").  Then you need to find FIRST commit of
v2.5 among

  $ git rev-list master --parents | grep -v ' '

The above finds all parent-less commits in 'master' branch; replace it
with branch holding v2.5 history.  Then you need to find LAST commit
of v2.4 and its SHA-1, e.g. via

  $ git rev-parse v2.4

Save current state of FIRST commit of v2.5 to a file

  $ git cat-file -p FIRST > tmp

Edit this file, adding 'parent' line between 'tree' and 'author'
headers, so the header of this file looks like the following:

  tree 13d050266e05f7c66000240814199fcf3b559d43
  parent ada9983c4256f5a7bac1f7f0e29d52011741d6aa
  author Jakub Narebski <[email-masked]> 1294231771 +0100

(trailing space added for better readibility).

Then you need to add newly created object to repository:

  $ git hash-object -t commit -w tmp

and then use it as replacement

  $ git replace <SHA-1 of FIRST> <SHA-1 returned by hash-object>

Finally check that replacement works, e.g.:

  $ git show FIRST
  $ git log --graph --oneline -3 FIRST

The anyone who would fetch refs/replace/ would get joined history, and
who doesn't would see it not connected.

P.S. This probably should made it into Documentation/howto

-- 
Jakub Narebski
Poland
ShadeHawk on #git