Merge branch 'jc/optional-path'

Merge branch 'jc/optional-path'

Configuration variables that take a pathname as a value
(e.g. blame.ignorerevsfile) can be marked as optional by prefixing
":(optoinal)" before its value.

* jc/optional-path:
  parseopt: values of pathname type can be prefixed with :(optional)
  config: values of pathname type can be prefixed with :(optional)
  t7500: fix GIT_EDITOR shell snippet
  t7500: make each piece more independent