aosp-merger: Improve merge commit message

* Merge and commit separately,
  to get newline between commit content and Change-Id
* Install prepare-commit-msg git hook, for nicer conflict messages
  https://gist.github.com/mikeNG/e55ef8e7a1e5cd596a950386503054fd/

Change-Id: I97370b352ccf4f146b4b6ea89af75e032fb13730

aosp-merger/prepare-commit-msg

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+case "$2,$3" in
+  merge,)
+    if grep -q "# It looks like you may be committing a merge." "$1"; then
+        sed -i -e '/# ------------------------ >8 ------------------------/,+3d' "$1"
+        /usr/bin/perl -i.bak -ne 's/^#// if /^# Conflicts/ .. /^#\R/; print' "$1"
+        sed -i '/ It looks like you may be committing a merge./i # ------------------------ >8 ------------------------' "$1"
+    fi
+    ;;
+
+  *)
+    ;;
+esac