Subject: bug#50762: diff-update-on-the-fly doesn't update hunk headers while regexp-replacing lines

From: Konstantin Kharlamov <[email-masked]>
Date: Thu, 23 Sep 2021 18:56:48 +0300

As title says. I think due to this bug it would make sense to have diff-update-on-the-fly set to nil by default.

# Steps to reproduce

1. In Emacs repo, `git checkout 72f8a9dd08132119a` (arbitrary commit for the sake of testcase) 
2. Create a patch from Emacs repo with `git format-patch -1 --stdout > /tmp/1.patch`, open it.
3. Note that 1st hunk has value `-2883,10 +2883,13`
4. Use regexp-replace aka `C-M-%` with `(.*^J\+.* → line1` (please replace ^J here with C-q C-j)
5. When asked for matches, press `!`, so all lines are replaced
6. Look at the first hunk again

## Expected

It changes its value to recalculation that should be happening

## Actual

It still has the older value, no recalculation has happened.