dev/ci: treat all Markdown diffs as docs changes

Created by: bobheadxi

A lot of Markdown files are in .prettierignore so they get skipped (for example, all of doc/ has been ignored for many years), but some aren't, so just to be safe let's make sure we run docs checks (which includes prettier) over everything on all Markdown diffs.

Also includes some other minor nitpicking changes to the diff detector

Test plan

n/a