Merge branch 'ds/config-literal-value'

Merge branch 'ds/config-literal-value'

Various subcommands of "git config" that takes value_regex
learn the "--literal-value" option to take the value_regex option
as a literal string.

* ds/config-literal-value:
  config doc: value-pattern is not necessarily a regexp
  config: implement --fixed-value with --get*
  config: plumb --fixed-value into config API
  config: add --fixed-value option, un-implemented
  t1300: add test for --replace-all with value-pattern
  t1300: test "set all" mode with value-pattern
  config: replace 'value_regex' with 'value_pattern'
  config: convert multi_replace to flags