[PATCH 1/2] builtin/rebase.c: Emit warning when rebasing without a forkpoint

A couple of years ago I submitted d1e894c6d7 (Document `rebase.forkpoint` in rebase man page, 2021-09-16) and during that discussion there was some talk about the behaviour of `git rebase'[1]. During that time I found that the documentation update was suffice. I wouldn't say it kept me awake at night but I do think that `git rebase' with or without an upstream supplied should behave the same in regards to forkpoints. This patch series addresses this behaviour change. It introduces a warning so users will have to set `rebase.forkpoint' in their configuration. In the future we can remove the warning and opt to pick `--no-fork-point' as a default value for `git rebase'.

There is one point where I'm a little confused, the `test_cmp' function in the testsuite doesn't like the output that is captured from STDERR, it seems that there is a difference in regards to whitespace. My workaround is to use `diff -wq`. I don't know if this is an accepted solution.

Another point of interest is that `git rebase' outputs `Successfully rebased and updated refs/heads/foo.' on STDERR and when everything is up to date it outputs `Current branch foo is up to date.' on STDOUT. I was a little confused by this. Especially since the output on STDOUT can be compared with `test_cmp'.

[1] https://lore.kernel.org/git/xmqqmtocrxwq.fsf@gitster.g/