t7063: work around FreeBSD's lazy mtime update feature

Let's start with the commit message of [1] from freebsd.git [2]

    Sync timestamp changes for inodes of special files to disk as late
    as possible (when the inode is reclaimed).  Temporarily only do
    this if option UFS_LAZYMOD configured and softupdates aren't
    enabled.  UFS_LAZYMOD is intentionally left out of
    /sys/conf/options.

    This is mainly to avoid almost useless disk i/o on battery powered
    machines.  It's silly to write to disk (on the next sync or when
    the inode becomes inactive) just because someone hit a key or
    something wrote to the screen or /dev/null.

    PR:             5577 [3]

The short version of that, in the context of t7063, is that when a
directory is updated, its mtime may be updated later, not
immediately. This can be shown with a simple command sequence

    date; sleep 1; touch abc; rm abc; sleep 10; ls -lTd .

One would expect that the date shown in `ls` would be one second from
`date`, but it's 10 seconds later. If we put another `ls -lTd .` in
front of `sleep 10`, then the date of the last `ls` comes as
expected. The first `ls` somehow forces mtime to be updated.

t7063 is really sensitive to directory mtime. When mtime is too "new",
git code suspects racy timestamps and will not trigger the shortcut in
untracked cache, in t7063.24 and eventually be detected in t7063.27

We have two options thanks to this special FreeBSD feature:

1) Stop supporting untracked cache on FreeBSD. Skip t7063 entirely
   when running on FreeBSD

2) Work around this problem (using the same 'ls' trick) and continue
   to support untracked cache on FreeBSD

I initially wanted to go with 1) because I didn't know the exact
nature of this feature and feared that it would make untracked cache
work unreliably, using the cached version when it should not.

Since the behavior of this thing is clearer now. The picture is not
that bad. If this indeed happens often, untracked cache would assume
racy condition more often and _fall back_ to non-untracked cache code
paths. Which means it may be less effective, but it will not show
wrong things.

This patch goes with option 2.

PS. For those who want to look further in FreeBSD source code, this
flag is now called IN_LAZYMOD. I can see it's effective in ext2 and
ufs. zfs is not affected.

[1] 660e6408e6df99a20dacb070c5e7f9739efdf96d
[2] git://github.com/freebsd/freebsd.git
[3] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=5577

Reported-by: Eric Wong <e@80x24.org>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7063-status-untracked-cache.sh

@@ -4,6 +4,20 @@ test_description='test untracked cache'
 
 . ./test-lib.sh
 
+# On some filesystems (e.g. FreeBSD's ext2 and ufs) directory mtime
+# is updated lazily after contents in the directory changes, which
+# forces the untracked cache code to take the slow path.  A test
+# that wants to make sure that the fast path works correctly should
+# call this helper to make mtime of the containing directory in sync
+# with the reality before checking the fast path behaviour.
+#
+# See <20160803174522.5571-1-pclouds@gmail.com> if you want to know
+# more.
+
+sync_mtime () {
+	find . -type d -ls >/dev/null
+}
+
 avoid_racy() {
 	sleep 1
 }
@@ -416,7 +430,8 @@ test_expect_success 'create/modify files, some of which are gitignored' '
 	echo four >done/four && # four is gitignored at a higher level
 	echo five >done/five && # five is not gitignored
 	echo test >base && #we need to ensure that the root dir is touched
-	rm base
+	rm base &&
+	sync_mtime
 '
 
 test_expect_success 'test sparse status with untracked cache' '