disallow ../ everywhere

This should have been done as soon as I rolled in support for "ua"
(unrestricted arguments; see comments in src/commands/git-annex-shell for
details).

Please also see mailing list thread around this date for more.  In particular,
I need to ackowledge someone who pointed this out in the context of
"symbolic-ref", but whose name/email I appear to have misplaced!
v3.6.2
(minor) fixup due to some (fedora??) path change
(minor) another test fixup

why was I doing this manually till now?
(minor) README fixup