Merge branch 'ta/config-add-to-empty-or-true-fix'

Merge branch 'ta/config-add-to-empty-or-true-fix'

"git config --add section.var val" used to lose existing
section.var whose value was an empty string.

* ta/config-add-to-empty-or-true-fix:
  config: avoid a funny sentinel value "a^"
  make config --add behave correctly for empty and NULL values