restore: default to HEAD when combining --staged and --worktree

restore: default to HEAD when combining --staged and --worktree

By default, files are restored from the index for --worktree, and from
HEAD for --staged. When --worktree and --staged are combined, --source
must be specified to disambiguate the restore source[1], thus making it
cumbersome to restore a file in both the worktree and the index.

However, HEAD is also a reasonable default for --worktree when combined
with --staged, so make it the default anytime --staged is used (whether
combined with --worktree or not).

[1]: Due to an oversight, the --source requirement, though documented,
is not actually enforced.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>