refs: clean up common_list

refs: clean up common_list

Instead of common_list having formatting like ! and /, use a struct to
hold common_list data in a structured form.

We don't use 'exclude' yet; instead, we keep the old codepath that
handles info/sparse-checkout and logs/HEAD.  Later, we will use exclude.

[jc: with "make common_list[] static" clean-up from Ramsay squashed in]

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>