clone: handle empty config values in -c

clone: handle empty config values in -c

"git clone --config" uses the following incantation to add an item to
a config file, instead of replacing an existing value:

	git_config_set_multivar_gently(key, value, "^$", 0)

As long as no existing value matches the regex ^$, that works as
intended and adds to the config.  When a value is empty, though, it
replaces the existing value.

Noticed while trying to set credential.helper during a clone to use a
specific helper without inheriting from ~/.gitconfig and
/etc/gitconfig.  That is, I ran

	git clone -c credential.helper= \
		-c credential.helper=myhelper \
		https://example.com/repo

intending to produce the configuration

	[credential]
		helper =
		helper = myhelper

Without this patch, the 'helper =' line is not included and the
credential helper from /etc/gitconfig gets used.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>