[BUG?] retrying with "am -3" doesn't work anymore

I noticed that I could not get a patch from Junio to apply earlier
today, and I think it is a regression in the builtin git-am
implementation.  I had trouble reproducing with a basic test case,
though.

Basically, I picked up the three patches from this sub-thread:

  http://thread.gmane.org/gmane.comp.version-control.git/288987/focus=290222

and tried to apply them on top of v2.8.0.

Doing it with "git am -3 patches" works. But doing it with:

  git am patches
  git am -3

doesn't. Bisecting shows that it did work before 783d7e8 (builtin-am:
remove redirection to git-am.sh, 2015-08-04).

I tried to tweak t4150 to show this by replacing a "git am -3 foo" with
"test_must_fail git am foo && git am -3", but it failed even on older
git, which made me think I fumbled the test somehow.

-Peff