clone: use a more appropriate variable name for the default refspec

clone: use a more appropriate variable name for the default refspec

cmd_clone() declares two strbufs 'key' and 'value' on the same line,
suggesting that they are used to contruct a config variable's name and
value.  However, this is not the case: 'key' is used to construct the
names of multiple config variables, while 'value' is never used as a
value for any of those config variables, or for any other config
variable for that matter, but only to contruct the default fetch
refspec.

Let's rename 'value' to 'default_refspec' to make the intent clearer.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>