git merge with removed secrets in diff causes commit to fail

Steps to reproduce:
1. create git repo
2. commit some secrets on branch X
3. create branch Y which removes those secrets
3. merge Y into X: `git checkout X && git merge Y --no-ff` (a fast forward would happen for this simple case, let's skip that)

if it matters, `alias gm='git merge'`

Mac OSX Catalina, using git 2.25.0 creates a diff after the comments for a `git merge`:
The diff after the separator is NOT included in the actual commit message, but presumably because it is fed through STDIN, git-secrets refuses to commit. Additionally, `git merge --no-verify` does not seem to work (`git commit --no-verify` does work)

Reproduce the report with git merge --no-ff after committing and then removing a secret, using git-secrets on macOS or an equivalent setup. Start by tracing how the merge hook receives STDIN and handles the diff after the commit-message separator, then compare git merge --no-verify with git commit --no-verify. Done means the merge succeeds without scanning non-message diff content, while secret checks remain effective.

Written by the indexing model from the issue text.