text/plain part in emails escapes HTML characters

Summary

text/plain part in emails escapes HTML characters

Steps to reproduce

Have a project with 'emails on push' integration enabled. Add a new commit with some HTML special characters, e.g. <> (for example by adding a Signed-off-by to your commit msg) Push the commit, and the email will have a broken text/plain part.

This may be the same bug as gitlab-ce#31219, but that one is for CI so I'm opening a new bug in case it needs fixing in two different places.

Example Project

I reproduced this on gitlab.com with https://gitlab.com/pochu/security-tracker/commit/06ea2bb82799b73ef95688cbe8a9da28320841f1

What is the current bug behavior?

The text/plain part in commit emails escapes HTML characters. For example:

Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit
[...]
Signed-off-by: Emilio Pozuelo Monfort &lt;pochu@debian.org&gt;

What is the expected correct behavior?

Those characters don't need escaping in text/plain parts

Relevant logs and/or screenshots

See the current bug behavior above, copied from an email from gitlab.com

Output of checks

This bug happens on GitLab.com

Edited by Nick Thomas