How to make sure fixup commits are not merged into master branch

I use git commit --fixup (or --squash) a lot, especially during code reviews. Obviously these commits should eventually disappear after a git rebase --autosquash but it worries me that I might forget to rebase and merge these commits into master.

How can I ensure that either I cannot merge these into certain branches, or at least that certain branches cannot be pushed with these commits in them?

You could at least block any pushes containing fixup! with the pre-push hook below.

#! /usr/bin/perl

use strict;
use warnings;

use constant Z40 => '0' x 40;

my($remote,$url) = @ARGV;

my $abort_push = 0;
while (<STDIN>) {
  # <local ref> SP <local sha1> SP <remote ref> SP <remote sha1> LF
  my($lref,$lsha,$rref,$rsha) = split;

  if ($lsha eq Z40) {} # ignore deletes
  else {
    my $commit_range =
      $rsha eq Z40
        ? $lsha            # new branch: check all commits
        : "$rsha..$lsha";  # existing: check new commits since $rsha
    my @cmd = (qw/ git rev-list --pretty=oneline --grep ^fixup! /, $commit_range);

    open my $fh, "-|", @cmd or die "$0: failed to start git rev-list: $!";
    my @fixup_commits;
    while (<$fh>) { push @fixup_commits, "  - $_" }
    close $fh;

    if (@fixup_commits) {
      my $s = @fixup_commits == 1 ? "" : "s";
      warn "Remove fixup$s from $lref:\n", @fixup_commits;
      $abort_push = 1;
    }
  }
}

die "Push aborted.\n" if $abort_push;

So for example with a history of

$ git lola
* 4a732d4 (HEAD -> feature/foo) fixup! fsdkfj
| * 478075c (master) w00t
| * 1d572d3 fixup! sdlkf
| * f9a55ee fixup! yo
|/  
* ea708b0 (origin/master) three
* d4276a2 two
* 6426569 hello

Attempting to push gives

$ git push origin master feature/foo
Remove fixups from refs/heads/master:
  - 1d572d32f963d6218ed3b92f69d58a8ec790d7ea fixup! sdlkf
  - f9a55ee14f28f9496e2aea1bc400ca65ae150f4b fixup! yo
Remove fixup from refs/heads/feature/foo:
  - 4a732d4601012246986037437ac0c0bab39dd0a9 fixup! fsdkfj
Push aborted.
error: failed to push some refs to [...]

Note that git lola is a non-standard but highly useful alias. Add the following to your global .gitconfig.

[alias]
        lol = log --graph --decorate --pretty=oneline --abbrev-commit
        lola = log --graph --decorate --pretty=oneline --abbrev-commit --all