More mark-up updates to typeset strings that are expected to
literally typed by the end user in fixed-width font.

* mm/doc-tt:
  doc: typeset HEAD and variants as literal
  CodingGuidelines: formatting HEAD in documentation
  doc: typeset long options with argument as literal
  doc: typeset '--' as literal
  doc: typeset long command-line options as literal
  doc: typeset short command-line options as literal
  Documentation/git-mv.txt: fix whitespace indentation