Sanity-check config variable names when adding and retrieving them.  As a side
effect code duplication between git_config_set_multivar and get_value (in
builtin/config.c) was removed and the common functionality was placed in
git_config_parse_key.

This breaks a test in t1300 which used invalid section-less keys in the tests
for "git -c". However, allowing such names there was useless, since there was
no way to set them via config file, and no part of git actually tried to use
section-less keys. This patch updates the test to use more realistic examples
as well as adding its own test.
Signed-off-by: NLibor Pechacek <lpechacek@suse.cz>
Acked-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>