doc: typeset HEAD and variants as literal

doc: typeset HEAD and variants as literal

This is an application of the newly added CodingGuidelines to HEAD and
variants like FETCH_HEAD. It was obtained with:

  perl -pi -e "s/'([A-Z_]*HEAD)'/\`\$1\`/g" *.txt

Signed-off-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>