pw-pull: use three-way merge for git-am

When test-applying a series to see if the contents of the patches
on the mailing list match the PR we can allow -3, this avoids
series application failing while the PR pulls cleanly.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
diff --git a/pw-pull b/pw-pull
index 7b9bfa3..90dbd3f 100755
--- a/pw-pull
+++ b/pw-pull
@@ -92,7 +92,7 @@
 if [ ! -z "$series" ]; then
     mbox_from_series $series
     git checkout -b $series_branch
-    git am mbox.i
+    git am -3 mbox.i
     rm -f mbox.i
     git checkout master
 fi