sporadic git failures on interactive rebase

Hi,

(git version 2.2.0)

I am currently developing/testing a script for a "history surgery" on a quite big repository (~30000 commits). The script always runs against exactly the same copy of a git repository. So things should be reproducable, but sometimes i get failures for the following sequence of commands:  

$ git checkout some_branch $ GIT_SEQUENCE_EDITOR="sed -i '1s/^pick /edit /'" git rebase -i $MERGETARGET~1 $ git rm -rf some_files $ git commit --amend --no-edit -c $MERGETARGET $ git rebase --continue

(where MERGETARGET is a valid commit id in the history of some_branch)

Here is an example output where things went wrong (MERGETARGET is 6185ac39299a740dc9bc6c5906dd1f229b3f046b). The interesting parts are the messages "c4095c1: not a commit that can be picked" and "error: short SHA1 c4095c1 is ambiguous.":

. Switched to branch 'master-flat' . c4095c1: not a commit that can be picked . 80f99bd: not a commit that can be picked . Stopped at 6185ac39299a740dc9bc6c5906dd1f229b3f046b... some_comment . You can amend the commit now, with . . git commit --amend . . Once you are satisfied with your changes, run . . git rebase --continue . rm 'some_files' . [detached HEAD 56675a06316345ac121997dde2b9eddb649d0539] some_comment . Author: user <foo.bar@com.com> . Date: Wed Sep 26 09:11:17 2012 +0000 . [more info about that commit] . error: short SHA1 c4095c1 is ambiguous. . fatal: Needed a single revision . Invalid commit name: c4095c1

Now that the command failed, i checked for ambigous c4095c1. But there is only one: $ git log -1 c4095c1 . commit c4095c1f5e7c126accf93ba68e2fa72bb055f567 . ...

Just as a test: $ git log -1 c409 . error: short SHA1 c409 is ambiguous. . error: short SHA1 c409 is ambiguous.

As i said above this issue is not strictly reproducable. I have a full backup of the working dir including the .git folder from just before the command sequence above. If i rollback and execute the commands again, everything works fine. Up to now i didn't had the situation the the sequence failed twice two in a row.

I also have the backup of the directory from directly after the sequence failure. So i can still test some commands if you need more information.

Does anybody have an idea of what's going on here?

Best regards Henning