git rebase --committer-date-is-author-date --root does not work

But git rebase --committer-date-is-author-date --root does not set the committer date to the author date for some reason.

Actually, it might set it correctly, starting from Git 23.19 (Q3 2018)

The "author-script" file "git rebase -i" creates got broken when we started to move the command away from shell script, which is getting fixed now.

See commit 5522bba, commit 67f16e3, commit 0f16c09, commit ca3e182 (31 Jul 2018) by Eric Sunshine (sunshineco).
^{(Merged by Junio C Hamano -- gitster -- in commit 1bc505b, 17 Aug 2018)}

sequencer: fix "rebase -i --root" corrupting author header timestamp

When "git rebase -i --root" creates a new root commit, it corrupts the "author" header's timestamp by prepending a "@":

author A U Thor <[email protected]> @1112912773 -0700

The commit parser is very strict about the format of the "author" header, and does not allow a "@" in that position.

The "@" comes from GIT_AUTHOR_DATE in "rebase-merge/author-script", signifying a Unix epoch-based timestamp, however, read_author_ident() incorrectly allows it to slip into the commit's "author" header, thus corrupting it.

One possible fix would be simply to filter out the "@" when constructing the "author" header timestamp, however, a more correct fix is to parse the GIT_AUTHOR_DATE date (via parse_date()) and format the parsed result into the "author" header.
Since "rebase-merge/author-script" may be edited by the user, this approach has the extra benefit of catching other potential timestamp corruption due to hand-editing.

We can do better than calling parse_date() ourselves and constructing the "author" header manually, however, by instead taking advantage of fmt_ident() which does this work for us.

The bad news

Unfortunately git rebase --root uses the interactive rebase code (because the non-interactive code cannot "replay" the root commit), and --committer-date-is-author-date is actually a flag passed to git am, which implements the simple non-interactive cases.

The good news

What git rebase does, at a fundamental level, is copy some commits (with, usually, some sort of change made during the copying process), then point a branch name at the final such copied commit. If there is just one commit you want to change-while-copying, you can use git commit --amend instead of git rebase.¹ If there is only one commit in the entire repository, there can only be one commit that you need to change-while-copying, so this case will apply.

Instead of --committer-date-is-author-date, you will need to use the GIT_COMMITTER_DATE variable to set the commit time stamp to some arbitrary value. You can also use --author and/or --date to override the author name and/or time-stamp. Hence:

t='2017-09-01 12:34:56'
GIT_COMMITTER_DATE="$t" git commit --amend --date="$t"

would set both time stamps to September 1st of 2017, at 12:34:56. (I used a shell variable t here to avoid typing in the same time stamp twice.)

(Add --no-edit if you don't want to edit the commit message. Remember that the new commit will use whatever is currently in the index! If you have changed the index since extracting the HEAD commit, you may want to copy the HEAD commit to a temporary index first, and use that.)

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¹This assumes the change you want to make is, e.g., the commit message text or date or author or some such, rather than the commit's parent ID. The definition of a root commit is one with no parent ID, and git commit --amend will continue to have no parent ID, which is what you want in this case.

TL;DR just give me a command line that works

git filter-branch --env-filter 'export GIT_COMMITTER_DATE="$GIT_AUTHOR_DATE"'