checkout: automerge local changes while switching branches.

When switching branches, if the working tree has a local
modification at paths that are different between current and new
branches, we refused the operation saying "cannot merge."  This
attempts to do an automerge for such paths.

This is still experimental.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-checkout.sh

@@ -121,7 +121,33 @@ then
 	git-checkout-index -q -f -u -a
 else
     git-update-index --refresh >/dev/null
-    git-read-tree -m -u $old $new
+    git-read-tree -m -u $old $new || (
+	echo >&2 -n "Try automerge [y/N]? "
+	read yesno
+	case "$yesno" in [yY]*) ;; *) exit 1 ;; esac
+
+	# NEEDSWORK: We may want to reset the index from the $new for
+	# these paths after the automerge happens, but it is not done
+	# yet.  Probably we need to leave unmerged ones alone, and
+	# yank the object name & mode from $new for cleanly merged
+	# paths and stuff them in the index.
+
+	names=`git diff-files --name-only`
+	case "$names" in
+	'')	;;
+	*)
+		echo "$names" | git update-index --remove --stdin ;;
+	esac
+
+	work=`git write-tree` &&
+	git read-tree -m -u $old $work $new || exit
+	if result=`git write-tree 2>/dev/null`
+	then
+	    echo >&2 "Trivially automerged." ;# can this even happen?
+	    exit 0
+	fi
+	git merge-index -o git-merge-one-file -a
+    )
 fi
 
 #