sys.exit(1) from makemigrations if no changes found

[Tim Heap]

It would be very useful for continuous deployment, testing, commit hooks, and other applications if django-admin makemigrations signaled via an exit code if any migrations were found. Commits in projects could be rejected if migrations were outstanding, continuous deployment systems could fail the build on outstanding migrations, and potentially other uses. No more would hasty commits break things when developers forgot to make migrations!

Changes to the code to make this happen are easy enough, but I am unsure how the command should behave. The grep unix utility is a example to copy. Under normal operation, grep always exits 0 unless an error happens, regardless of whether it found any matches. Invoking grep with the -q/--quiet flag causes grep to be silent, not printing anything, as well as exiting 0 if matches are found and 1 if nothing is found.

I am proposing django-admin makemigrations should exit with 1 (or anything non-zero) if no migrations to make were found, or exit 0 if migrations to make were found. As the command is instructed to make migrations, not making any is the error case.

I am unsure how this new functionality should be selected by the user when invoking makemigrations. The options I see are:

1. Enable this always. This is very simple to implement and easy to understand. Good unixy tools use error codes extensively to signal errors. This may be surprising behaviour when compared to grep though, and breaks backwards compatibility in a minor way.
2. Enable this when the --dry-run flag is enabled. Now this flag can be used to check for migrations that need to be created both visually via the printed text, and composed in shell commands.
3. Add a new flag -e/--exit (or similar). The sole purpose of this flag would be to exit with 1 when no migrations were found. This could be combined with --dry-run to just check for migrations that need to be made.
4. Add a new flag -q/--quiet that copies the behaviour of greps -q/--quiet flag: silences output and exits with 1 when no migrations were found. This duplicates functionality though, as logging can be silenced using -v0 already.

My personal preference is for option 2. I was surprised when enabling --dry-run did not signal its result via the exit code. 3 would be the cleanest and most composable option.

I will implement this change using 2, unless other people have opinions on the matter.