git-am: -i does not take a string parameter.

$ git am -3 -s -i file

spewed the usage strings back at the user while

    $ git am -3 -i -s file

didn't.  This fixes it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -9,7 +9,7 @@ git-am [options] --resolved
 git-am [options] --skip
 --
 d,dotest=       use <dir> and not .dotest
-i,interactive=  run interactively
+i,interactive   run interactively
 b,binary        pass --allo-binary-replacement to git-apply
 3,3way          allow fall back on 3way merging if needed
 s,signoff       add a Signed-off-by line to the commit message