Rebase instruction include the deprecated `--preserve-merges` option

[choznerol]

bors/homu suggest using git rebase upstream/master -p

Example

https://github.com/rust-lang/cargo/pull/14078#issuecomment-2170936637

But -p (or --rebase-merges) has been removed in git 2.34.0 back in 2021.

* The "--preserve-merges" option of "git rebase" has been removed.

so you get:

$ git rebase upstream/master -p
fatal: --preserve-merges was replaced by --rebase-merges
Note: Your `pull.rebase` configuration may also be set to 'preserve',
which is no longer supported; use 'merges' instead

$ git rebase upstream/master --preserve-merges
fatal: --preserve-merges was replaced by --rebase-merges
Note: Your `pull.rebase` configuration may also be set to 'preserve',
which is no longer supported; use 'merges' instead

Since the change is trivial, I'll just make a PR to update it here:

https://github.com/rust-lang/homu/blob/f652a9e7ecc42a1a590f48e96d9d34df4596aedb/homu/main.py#L950

Reference:

• Similar issue: Git no longer supports 'preserve-merges', stop offering that option
• Old doc that still include --preserve-merges: https://git-scm.com/docs/git-rebase/2.28.0#Documentation/git-rebase.txt---preserve-merges