core.fsyncmethod: tests for batch mode

core.fsyncmethod: tests for batch mode

Add test cases to exercise batch mode for:
 * 'git add'
 * 'git stash'
 * 'git update-index'
 * 'git unpack-objects'

These tests ensure that the added data winds up in the object database.

In this change we introduce a new test helper lib-unique-files.sh. The
goal of this library is to create a tree of files that have different
oids from any other files that may have been created in the current test
repo. This helps us avoid missing validation of an object being added
due to it already being in the repo.

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