revision.h: new flag in struct rev_info wrt. worktree-related refs

revision.h: new flag in struct rev_info wrt. worktree-related refs

The revision walker can walk through per-worktree refs like HEAD or
SHA-1 references in the index. These currently are from the current
worktree only. This new flag is added to change rev-list behavior in
this regard:

When single_worktree is set, only current worktree is considered. When
it is not set (which is the default), all worktrees are considered.

The default is chosen so because the two big components that rev-list
works with are object database (entirely shared between worktrees) and
refs (mostly shared). It makes sense that default behavior goes per-repo
too instead of per-worktree.

The flag will eventually be exposed as a rev-list argument with
documents. For now it stays internal until the new behavior is fully
implemented.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>