How to change git commit message without changing commit hash

Question

The title is not exact, but I can't express it better in a single line.

• I actually know how to change git commit message like here.
• But I know it always changes the SHA-1 too, which I want to avoid.

I only want to see a different message in git-log. I thought it could be done somehow using git-notes, but I haven't managed it.

---

Explanation:

I need it in order to fix errors in the commit messages. I always write there the name of a document containing my communication with the customer (it looks just like T1234 Replace foo by bar). The communication tends to be quite long, so I can loose a lot of time till I find out I was being mislead by wrong document name in the commit message.

Use of git-notes

It looks like git-notes in fact works as stated here. However I always use

git log --oneline

so I never see it. Concerning the comment about making git lie to the user: IMHO, this would be acceptable when this happened only when using a special switch like --replace-messages-by-notes, wouldn't it? As I always use an alias instead of using git log directly, I'd get what I want without typing a lot.

Do you think it's a reasonable feature request or would you recommend another workflow to me?

Answer 1

As various people have pointed out (e.g. in VonC's very useful answer), git notes really is the mechanism you're looking for. Is it not enough for you to change your alias to the following?

git log --oneline --show-notes

Presumably it's only occasionally that you'll have to add a note to a commit, and the notes will visually stand out in the output of that command.

If you really want to replace the subject of each commit if notes exist, you could always create a script along the lines of:

for c in $(git rev-list HEAD)
do
    n=$(git notes show $c 2> /dev/null)
    m=$(git show --oneline $c|head -1)
    if [ -n "$n" ]
    then
       m=${m/ */ $n}
    fi
    echo $m
done

... but that's a lot uglier for little gain, in my opinion.

Answer 2

git notes is the only way to have a different git log message (different than the commit message) without changing the SHA1, as mentioned in the "Notes to Self" article.

A few remarks though:

• Notes are organized by namespace, the default one being "commits".
• Notes don't modify the commit message, they only add to it (which might be why git notes isn't working for you).
• Notes aren't pushed by default, unless you specify explicitly the refspec for them (refs/notes/*)

Answer 3

Technically this seems impossible (at least to me, I'm not a git pro though).

A git commit stores a tree hash (think: the state of your working directory at that time) with additional commit information. When you change the commit message, the tree hash won't change, however the commit hash will change since it is computed from the commit object, there's no way around it.

See Progit internals for details.