test-lib-functions: add parsing helpers for ls-files and ls-tree

test-lib-functions: add parsing helpers for ls-files and ls-tree

Several tests use awk to parse OIDs from the output of 'git ls-files
--stage' and 'git ls-tree'. Introduce helpers to centralize these uses
of awk.

Update t5317-pack-objects-filter-objects.sh to use the new ls-files
helper so that it has some usages to review. Other updates are left for
the future.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>