Make git config variable names case-insensitive

Make git config variable names case-insensitive

They always were meant to be case-insensitive, but I had missed one
"tolower()", making that not true.

The actual _values_ aren't case-insensitive, of course, although some uses
of them may be (ie boolean parsing uses "strcasecmp()" to match against
the strings "true" and "false").

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>