Merge branch 'jc/config-case-cmdline-take-2'

The code to parse "git -c VAR=VAL cmd" and set configuration
variable for the duration of cmd had two small bugs, which have
been fixed.

* jc/config-case-cmdline-take-2:
config: use git_config_parse_key() in git_config_parse_parameter()
config: move a few helper functions up

diff --cc config.c
Simple merge

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