dir.c: use a single struct exclude_list per source of excludes

dir.c: use a single struct exclude_list per source of excludes

Previously each exclude_list could potentially contain patterns
from multiple sources.  For example dir->exclude_list[EXC_FILE]
would typically contain patterns from .git/info/exclude and
core.excludesfile, and dir->exclude_list[EXC_DIRS] could contain
patterns from multiple per-directory .gitignore files during
directory traversal (i.e. when dir->exclude_stack was more than
one item deep).

We split these composite exclude_lists up into three groups of
exclude_lists (EXC_CMDL / EXC_DIRS / EXC_FILE as before), so that each
exclude_list now contains patterns from a single source.  This will
allow us to cleanly track the origin of each pattern simply by adding
a src field to struct exclude_list, rather than to struct exclude,
which would make memory management of the source string tricky in the
EXC_DIRS case where its contents are dynamically generated.

Similarly, by moving the filebuf member from struct exclude_stack to
struct exclude_list, it allows us to track and subsequently free
memory buffers allocated during the parsing of all exclude files,
rather than only tracking buffers allocated for files in the EXC_DIRS
group.

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>