support boolean git configs that are represented by the name of the setting with...

support boolean git configs that are represented by the name of the setting with no value

Eg"core.bare" is the same as "core.bare = true".

Note that git treats "core.bare =" the same as "core.bare = false", so the
code had to become more complicated in order to treat the absense of a
value differently than an empty value. Ugh.