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

"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

diff --cc builtin/config.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc config.c
Simple merge