Use merge-recursive in git-am -3.

By switching from merge-resolve to merge-recursive in the 3-way
fallback behavior of git-am we gain a few benefits:

 * renames are automatically handled, like in rebase -m;
 * conflict hunks can reference the patch name;
 * its faster on Cygwin (less forks).

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-am.sh

@@ -88,17 +88,20 @@ It does not apply to blobs recorded in its index."
     # This is not so wrong.  Depending on which base we picked,
     # orig_tree may be wildly different from ours, but his_tree
     # has the same set of wildly different changes in parts the
-    # patch did not touch, so resolve ends up canceling them,
+    # patch did not touch, so recursive ends up canceling them,
     # saying that we reverted all those changes.
 
-    git-merge-resolve $orig_tree -- HEAD $his_tree || {
+    eval GITHEAD_$his_tree='"$SUBJECT"'
+    export GITHEAD_$his_tree
+    git-merge-recursive $orig_tree -- HEAD $his_tree || {
 	    if test -d "$GIT_DIR/rr-cache"
 	    then
 		git-rerere
 	    fi
 	    echo Failed to merge in the changes.
 	    exit 1
     }
+    unset GITHEAD_$his_tree
 }
 
 prec=4