There's a closed pull request on the mblaze repository

mrep: fix subject not populating when pager stdout is not tty
https://github.com/leahneukirchen/mblaze/pull/243

Description:
If the pager is set to something where stdout is not a TTY, such as a text editor, it causes non-interactive calls to `mscan` to fail with the error: `Fatal error: stdout is not a tty`. When building the subject, `mrep` fetches the subject from the email being replied to using `mscan`, causing this error to manifest and the subject line to end up as: "Re: ". Explicitly setting the pager to `cat` when fetching the email subject in `mrep` prevents this error.