[BUG] Cannot push some grafted branches

There seems to be some bad interactions between git-push and grafts. The problem seems to occur when a commit that exists in the remote repo is subject to a graft in the local repo, and we try to push one of the fake parents.

The problem was first seen on 1.7.12.3 in a private repo, and I could reproduce it using 1.8.1.rc0, as shown below. 1.7.10.4 seems even more affected, with something looking like a memory corruption issue.

Here is the test:

$ git clone git.git git-test
Cloning into 'git-test'...
done.
Checking out files: 100% (2518/2518), done.
$ cd git-test/
git-test$ git co maint
Branch maint set up to track remote branch maint from origin.
Switched to a new branch 'maint'
git-test$ echo >> README 
git-test$ git commit -a -m "test"
[maint 0708279] test
 1 file changed, 1 insertion(+)
git-test$ echo $(git rev-parse origin/master; git rev-parse origin/master^; git rev-parse HEAD) > .git/info/grafts

git-test$ git version
git version 1.8.1.rc0
git-test$ git push origin maint
Total 0 (delta 0), reused 0 (delta 0)
fatal: bad object 0708279e168b52003234dd23601796b3b12e278b
fatal: bad object 0708279e168b52003234dd23601796b3b12e278b
To /home/localadm/softs/git.git
 ! [remote rejected] maint -> maint (missing necessary objects)
error: failed to push some refs to '/home/localadm/softs/git.git'

$ git version git version 1.7.10.4

git-test$ git push origin maint
Total 0 (delta 0), reused 0 (delta 0)
fatal: bad object 0708279e168b52003234dd23601796b3b12e278b
fatal: bad object 0708279e168b52003234dd23601796b3b12e278b
Auto packing the repository for optimum performance.
fatal: protocol error: bad line length character: Remo
error: error in sideband demultiplexer
error: ૏        >S��ŋJ�jB�;�x'��R died of signal 13
To /home/localadm/softs/git.git
 ! [remote rejected] maint -> maint (missing necessary objects)
error: failed to push some refs to '/home/localadm/softs/git.git'