stash: integrate with sparse index

stash: integrate with sparse index

Enable sparse index in 'git stash' by disabling
'command_requires_full_index'.

With sparse index enabled, some subcommands of 'stash' work without
expanding the index, e.g., 'git stash', 'git stash list', 'git stash drop',
etc. Others ensure the index is expanded either directly (as in the case of
'git stash [pop|apply]', where the call to 'merge_recursive_generic()' in
'do_apply_stash()' triggers the expansion), or in a command called
internally by stash (e.g., 'git update-index' in 'git stash -u'). So, in
addition to enabling sparse index, add tests to 't1092' demonstrating which
variants of 'git stash' expand the index, and which do not.

Finally, add the option to skip writing 'untracked.txt' in
'ensure_not_expanded', and use that option to successfully apply stashed
untracked files without a conflict in 'untracked.txt'.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>