Merging two git repositories to obtain linear history

Question

I have two git repositories and a lot of untracked changes between them:

   ftp -->            C-- (untracked changes) --D
                     /                           \
   git        A--B--C <-- old/master              \
                                                   \
                                                    \
                                  new/master -->     D--E--F 

How can I merge old repository into new repository to have a linear history like

A--B--C--D--E--F

EDIT:

inspired by How can I combine Git repositories into a linear history?

I've done:

git clone url://new new
cd new/
git remote add old url://old
git fetch old
git reset --hard origin/master
git filter-branch --parent-filter 'sed "s_^\$_-p old/master_"' HEAD
git push origin master

Only problem is that every commit from new/master was doubled (due to change of parent I think) so I've now (M is merge commit)

         D---E---F--         
                    \
A--B--C--D'--E'--F'--M 

How can I easily remove unnecessary commits (D - F and maybe M)?

Answer 1

Fixing the results of git filter-branch

If you have a repository that looks like this:

         D---E---F--
                    \
A--B--C--D'--E'--F'--M <-master

and you want the result to look like this:

A--B--C--D'--E'--F' <-master

then you can simply force master to point to F':

git checkout master
git reset --hard <sha1-of-F'>

This will cause commits D, E, F, and M to become unreachable, effectively deleting them (they will be garbage collected after a while).

Starting over from scratch

Assuming you have two repositories that look like this:

• old: A--B--C <-master
• new: D--E--F <-master

and you want the result to be:

• combined: A--B--C--D'--E'--F' <- master

then you can perform the following steps:

1. Initialize the combined repository:

   git init combined
   cd combined
   git remote add old url:/to/old
   git remote add new url:/to/new
   git remote update
   

   At this point your combined repository looks like:

   A--B--C <-old/master
   
   D--E--F <-new/master
   

   Note that the two branches aren't connected in any way.

2. Set your master branch to point to C:

   git reset --hard old/master
   

   Now your repository looks like this:

         old/master
         |
         v
   A--B--C <-master
   
   D--E--F <-new/master
   

3. Find the sha1 of D:

   d=$(git rev-list --reverse new/master | head -n 1)
   

4. Import D into your working directory and index by reading the contents of the commit

   git read-tree -u --reset $d
   

5. commit the contents of D using the same commit message, author, date, etc. as the original D commit:

   git commit -C $d
   

   Now your repository looks like this:

         old/master
         |
         v
   A--B--C--D' <-master
   
   D--E--F <-new/master
   

6. cherry-pick the rest of the commits:

   git cherry-pick $d..new/master
   

   Now your repository looks like this:

         old/master
         |
         v
   A--B--C--D'--E'--F' <-master
   
   D--E--F <-new/master
   

7. Clean up:

   git remote rm old
   git remote rm new
   

   Now your repository looks like this:

   A--B--C--D'--E'--F' <-master
   

Answer 2

Just check out your branch and run:

  git reset --hard **SHA-OF-F'**

That will remove M and D-F from your branch.