config: use git_config_parse_key() in git_config_parse_parameter()

config: use git_config_parse_key() in git_config_parse_parameter()

The parsing of one-shot assignments of configuration variables that
come from the command line historically was quite loose and allowed
anything to pass.  It also downcased everything in the variable name,
even a three-level <section>.<subsection>.<variable> name in which
the <subsection> part must be treated in a case sensitive manner.

Existing git_config_parse_key() helper is used to parse the variable
name that comes from the command line, i.e. "git config VAR VAL",
and handles these details correctly.  Replace the strbuf_tolower()
call in git_config_parse_parameter() with a call to it to correct
both issues.  git_config_parse_key() does a bit more things that are
not necessary for the purpose of this codepath (e.g. it allocates a
separate buffer to return the canonicalized variable name because it
takes a "const char *" input), but we are not in a performance-critical
codepath here.

Signed-off-by: Junio C Hamano <gitster@pobox.com>