[PATCH v2] new: Don't scan unchanged directories with no sub-directories

[David Bremner]

Austin Clements <amdragon at MIT.EDU> writes:

> This can substantially reduce the cost of notmuch new in some
> situations, such as when the file system cache is cold or when the
> Maildir is on NFS.

On my desktop at home (a core i7 950) with spinning rust disks (and lvm
on luks) this patch yields about a 7% slowdown in the intial new perf
test

from

			Wall(s)	Usr(s)	Sys(s)	Res(K)	In/Out(512B)
  Initial notmuch new   579.60	348.86	14.26	217188	5330266/3501272

to

			Wall(s)	Usr(s)	Sys(s)	Res(K)	In/Out(512B)
  Initial notmuch new   620.51	368.62	15.48	217156	5330354/3416456

On an SSD I don't detect a significant different (<0.5% speedup)

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