[ITCH] Mixed-case aliases

Hi,

My .gitconfig has aliases like the following:

[alias]
	bD = branch -D
	bM = branch -M

Unfortunately, since the current config-parsing code unconditionally
runs tolower() on all keys, these aliases don't work.  My proposal is
to modify alias.c::alias_lookup() to call a
git_config_DONTMANGLECASE() variant, and modify the various functions
in config.c appropriately.

On a related note, it might be useful to warn the user that we are an
ignoring alias.<builtin-name> key when executing the corresponding
builtin.  It's not really my itch, but I suspect a lot of beginners
get tripped up by this.

What do you think?

Ram