what does rebase -i do if nothing has changed? - git-rebase

Question

I noticed that if i do a
git rebase -i HEAD~10
and then don't change anything in the resulting file, some sort of rebase still gets performed. I.e. I need to
git push origin branch --force
since a regular push
git push origin branch
would apparently lose history!
This seems to imply that some history was changed but i don't understand why. If i didn't choose to change any commit (i.e. every commit was picked) then why did the history change and what specifically changed?

Answer 1

If you do a git log --format=fuller you'll see that a commit not only tracks the author, but the committer:
commit 0d987988ffed315ef63f79b33eae40a8715390c9
Author: Statichippo <example#example.com>
AuthorDate: Fri Jun 22 14:09:48 2012 -0400
Commit: Statichippo <example#example.com>
CommitDate: Fri Jun 22 14:09:48 2012 -0400
a commit message
When you rebase, your user is listed as the new committer. Here's an example of what it would look like if I rebased that code block:
commit 3e23bea3c778c66h59c5f665061e9354507b379b
Author: Statichippo <example#example.com>
AuthorDate: Fri Jun 22 14:09:48 2012 -0400
Commit: Jacob Angel <jangel#example.com>
CommitDate: Tue Jul 3 12:39:43 2012 -0400
a commit message
So what's probably happening is that you're rebasing commits that were made by another person, and it's changing the commit and CommitDate. Obviously git will see a new hash (because part of the commit's changed) and note the change.