Merge branch 'cn/config-missing-path'

"git config --path $key" segfaulted on "[section] key" (a boolean
"true" spelled without "=", not "[section] key = true").

* cn/config-missing-path:
config: don't segfault when given --path with a missing value

diff --cc builtin/config.c
Simple merge

diff --cc t/t1300-repo-config.sh
Simple merge