checkout: introduce --{,no-}overlay option

Currently 'git checkout' is defined as an overlay operation, which
means that if in 'git checkout <tree-ish> -- [<pathspec>]' we have an
entry in the index that matches <pathspec>, but that doesn't exist in
<tree-ish>, that entry will not be removed from the index or the
working tree.

Introduce a new --{,no-}overlay option, which allows using 'git
checkout' in non-overlay mode, thus removing files from the working
tree if they do not exist in <tree-ish> but match <pathspec>.

Note that 'git checkout -p <tree-ish> -- [<pathspec>]' already works
this way, so no changes are needed for the patch mode.  We disallow
'git checkout --overlay -p' to avoid confusing users who would expect
to be able to force overlay mode in 'git checkout -p' this way.

Untracked files are not affected by this change, so 'git checkout
--no-overlay HEAD -- untracked' will not remove untracked from the
working tree.  This is so e.g. 'git checkout --no-overlay HEAD -- dir/'
doesn't delete all untracked files in dir/, but rather just resets the
state of files that are known to git.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9902-completion.sh

@@ -1436,6 +1436,7 @@ test_expect_success 'double dash "git checkout"' '
 	--progress Z
 	--no-quiet Z
 	--no-... Z
+	--overlay Z
 	EOF
 '