Merge branch 'kb/p4merge'

Adjust the order mergetools feeds the files to the p4merge backend
to match the p4 convention.

* kb/p4merge:
  merge-one-file: force content conflict for "both sides added" case
  git-merge-one-file: send "ERROR:" messages to stderr
  git-merge-one-file: style cleanup
  merge-one-file: remove stale comment
  mergetools/p4merge: create a base if none available
  mergetools/p4merge: swap LOCAL and REMOTE

git-sh-setup.sh

@@ -249,6 +249,18 @@ clear_local_git_env() {
 	unset $(git rev-parse --local-env-vars)
 }
 
+# Generate a virtual base file for a two-file merge. Uses git apply to
+# remove lines from $1 that are not in $2, leaving only common lines.
+create_virtual_base() {
+	sz0=$(wc -c <"$1")
+	@@DIFF@@ -u -La/"$1" -Lb/"$1" "$1" "$2" | git apply --no-add
+	sz1=$(wc -c <"$1")
+
+	# If we do not have enough common material, it is not
+	# worth trying two-file merge using common subsections.
+	expr $sz0 \< $sz1 \* 2 >/dev/null || : >"$1"
+}
+
 
 # Platform specific tweaks to work around some commands
 case $(uname -s) in