Re: Alter parent ID of existing commit object

> Hi Paul,
>
> Paul Richards wrote:
>
>> Is it possible to edit an old commit object and only alter the ID of
>> the parent commit but otherwise leave all the other information intact
>> (tree, message, authors, date, etc).
>>
>> I'd expect such a command to return the new hash of the modified commit.
>
> The standard answer to this question is to say “use grafts and
> filter-branch”.  The git-filter-branch(1) man page explains this
> approach.  It is very powerful, but sometimes I do not want to have
> that much power.
>
> So I will tell a secret: in the scenarios when I wanted something like
> this (actually, what I have occasionally wanted is to transform a
> single-parent commit into a merge), I did something like the following:
>
>  $ git cat-file commit $rev
>  tree dcd2cc4b76f8756423f5c1ab7d2c62d458a8b15f
>  parent 5f1e6d9ce35e212708f9adc55e6b9a7e0d296df4
>  author Will Palmer <wmpalmer@gmail.com> 1272275407 -0500
>  committer Jonathan Nieder <jrnieder@gmail.com> 1272275443 -0500
>
>  pretty: Respect --abbrev option
>
>  Prior to this, the output of git log -1 --format=%h was always 7
>  characters long, without regard to whether --abbrev had been passed.
>
>  Signed-off-by: Will Palmer <wmpalmer@gmail.com>
>  Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
>  $ git cat-file commit $rev >tmp
>  $ sed -i -e "s/parent .*/parent $(git rev-parse othercommit)/" tmp
>  $ git hash-object -t commit -w tmp
>  ca55c560685284ac6d121939b2cd881f426e7074
>
> Easy.  Still, I would be happy to see this packaged in a command, so I
> could recommend it in combination with ‘git replace’ to people who are
> scared of sed.
>
> Thanks for bringing it up.
> Jonathan
>