I have been trying to apply a git patch that was not generated via git-format-patch(1) and which git has consequently failed to apply. The repository in question is publicly available here (https://github.com/acl2/acl2/) and the patch contents are publicly available here (https://utlists.utexas.edu/sympa/arc/acl2-help/2021-02/msg00016.html).
As you can see, the "patch" here is really the raw shell output of the git-diff(1) command. I started by downloading the email as a .eml file and attempting "git am" and "git apply". Both failed. I tried a few tweaks, including removing text past the end of the diff and pasting the diff contents into a different patch file that I know git to accept (and changing the diffstat just before the start of the diff to mention the correct files.) None of these tweaks worked either with the above two commands.
I'm aware this diff is small enough for it to take less time for me to apply it by hand than write to Stack Overflow, but something tells me this problem must have bothered someone else before and a solution must exist that actually uses git's patch application tools. Let me know - thanks!