find-backports: warn if any commits are not from the first reference

Every branch (for example "nm-1-40") has exactly one next branch, from
which patches should be backported (in that example that branch is
"nm-1-42").

While "find-backports" searches all newer branches for patches, it does
not make it clear form where the patch should come from.

That means, if you run the script `contrib/scripts/find-backports origin/nm-1-40`
it will check nm-1-42 and main branch, and might suggest to backport
patches that are only on main, but not "nm-1-42". That would be wrong,
because patches need to first go into nm-1-42, and then backported (from
there) further to nm-1-40.

Print a warning to highlight that.

contrib/scripts/find-backports

@@ -139,6 +139,11 @@ def git_all_commits(rnge):
     return git_all_commits_grep(rnge)
 
 
+@memoize
+def git_all_commits_set(rnge):
+    return set(git_all_commits_grep(rnge))
+
+
 def git_commit_sorted(commits):
     commits = list(commits)
     if not commits:
@@ -256,6 +261,10 @@ def git_commits_annotate_cherry_picked(rnge):
     return c_dict
 
 
+def git_ref_in_history(ref, rnge):
+    return git_ref_exists(ref) in git_all_commits_set(rnge)
+
+
 if __name__ == "__main__":
     if len(sys.argv) <= 1:
         ref_head0 = "HEAD"
@@ -371,5 +380,22 @@ if __name__ == "__main__":
 
     print_err(git_logg(commits_good))
 
+    not_in = [
+        c
+        for c in commits_good
+        if not git_ref_in_history(c, f"{ref_head}..{ref_upstreams[0]}")
+    ]
+    if not_in:
+        print_err("")
+        print_err(
+            f'WARNING: The following commits are not from the first reference "{ref_upstreams[0]}".'
+        )
+        print_err(
+            f'  You may want to first backports those patches to "{ref_upstreams[0]}".'
+        )
+        for l in git_logg(git_commit_sorted(not_in)).splitlines():
+            print_err(f"  - {l}")
+        print_err("")
+
     for c in reversed(commits_good):
         print("%s" % (c))