show-branch: --no-sparse should give dense output

show-branch: --no-sparse should give dense output

"git show-branch --no-sparse" behaved exactly the same way as "git
show-branch --sparse", which did not make any sense.  This was
because it used a variable "dense" initialized to 1 by default to
give "non sparse" behaviour, and OPT_SET_INT() to set the varilable
to 0 in response to the "--sparse" option.  Unfortunately,
OPT_SET_INT() sets 0 to the given variable when the option is
negated.

Flip the polarity of the variable "dense" by renaming it to "sparse"
and initializing it to 0, and have OPT_SET_INT() set the variable to
1 when "--sparse" is given.  This way, "--no-sparse" would set 0 to
the variable and would give us the "dense" behaviour.

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