Difficulties using git rebase. Help, please!

Hello, Git.

Some while ago I made some amendments to the Linux kernel for my own use
(at least).  I now want to rebase these changes onto the master branch of
the Linux Stable repository.

When I wrote the changes, I based them off branch linux-6.13.y.  The
command I tried to rebase with was, with PWD being the pertinent copy of
the Linux repository:

    $ git rebase --onto master origin/linux-6.13.y HEAD

..  This didn't work well.  In particular, I got a conflict in a file that
I had never changed.  Why?

Well, I corrected the conflicts in that file, git add'ed it, git rebase
--continue'd, then got another conflict in a file I'd never touched.
Same again.  After the third such conflict, I gave up with git rebase
--abort.

Criticism: there doesn't appear to be a --dry-run option in git rebase,
with which one can see how many files will be conflicted.  Instead they
are notified one at a time, drip, drip, drip, .... to the user.  In my
case there might have been four conflicted files, there might have been a
thousand.  Either I'm missing something, or git rebase is missing
something, hopefully the former.

Incidentally, when I do git status, I get as part of the output:

    Your branch is ahead of 'origin/linux-6.13.y' by 2012 commits.

..  I haven't done 2012 git commits in my life.  What does this number
2012 mean?

So, back to git rebase.  Would somebody please explain why I am seeing
these conflicts at all?  Please also help me make progress.  I'm stuck.

Thanks!

-- 
Alan Mackenzie (Nuremberg, Germany).