brew: brew upgrade breaks terminal, requiring reset

Bug report:

Note: I have described the issue on StackExchange. No answer so far, so I thought I’d post it here.

When I run brew upgrade in my Bash shell, it breaks the terminal display (in both iTerm2 and Terminal) such that I can’t see anything I type in the following prompt and I can’t see the history. For example, here is what my terminal looks like after typing brew upgrade\necho hello\n at an empty prompt:

$ brew upgrade
$ hello
$

The only way I’ve found to fix the current terminal is to use reset.

The following do not cause this problem:

$ brew upgrade 1> /dev/null # piping stdout
$ brew upgrade | cat

However, I still see the problem with:

$ brew upgrade 2> /dev/null # piping stderr

I don’t see the problem in zsh.

I do have a custom prompt, but the problem still exists after disabling it by removing my .bashrc.

When I check the output for ANSI color codes, I don’t see any.

The problem might have started when I upgraded to OS X from Yosemite to El Capitan (10.11). It’s been some time, so I don’t remember exactly.

I suppose the problem might be discovered by reviewing the current Homebrew upgrade.rb code, but I wouldn’t know where to start.