Merge branch 'jc/optional-path' into jch

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

Comments?

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

diff --cc Documentation/config.adoc
Simple merge

diff --cc config.c
Simple merge

diff --cc parse-options.c
Simple merge