Merge branch 'js/config-set-in-non-repository'

"git config section.var value" to set a value in per-repository
configuration file failed when it was run outside any repository,
but didn't say the reason correctly.

* js/config-set-in-non-repository:
git config: report when trying to modify a non-existing repo config

diff --cc builtin/config.c
Simple merge

diff --cc t/t1308-config-set.sh
Simple merge