Compatibility with .md files

This wiki is from a time in which the suffix .md was not yet popularised
Preserve history even when renamed, also rev-list is too verbose
Sanity checks & supporting features with old content suffixes

There's a low threshold for file-similarity detection (git diff -C0.1) -
in the wiki, there's no situation in which two files might be compared
that aren't actually the same
Merge branch 'multi-content-suffix'
Embarrassing bug resulting in 500 when no suffix-old is set