diff --git a/hooks/commit-msg b/hooks/commit-msg
index 70d67eaf..8c6476fa 100755
--- a/hooks/commit-msg
+++ b/hooks/commit-msg
@@ -1,5 +1,5 @@
 #!/bin/sh
-# From Gerrit Code Review 3.1.3
+# From Gerrit Code Review 3.6.1 c67916dbdc07555c44e32a68f92ffc484b9b34f0
 #
 # Part of Gerrit Code Review (https://www.gerritcodereview.com/)
 #
@@ -17,6 +17,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+set -u
+
 # avoid [[ which is not POSIX sh.
 if test "$#" != 1 ; then
   echo "$0 requires an argument."
@@ -29,15 +31,25 @@ if test ! -f "$1" ; then
 fi
 
 # Do not create a change id if requested
-if test "false" = "`git config --bool --get gerrit.createChangeId`" ; then
+if test "false" = "$(git config --bool --get gerrit.createChangeId)" ; then
   exit 0
 fi
 
-# $RANDOM will be undefined if not using bash, so don't use set -u
-random=$( (whoami ; hostname ; date; cat $1 ; echo $RANDOM) | git hash-object --stdin)
+# Do not create a change id for squash commits.
+if head -n1 "$1" | grep -q '^squash! '; then
+  exit 0
+fi
+
+if git rev-parse --verify HEAD >/dev/null 2>&1; then
+  refhash="$(git rev-parse HEAD)"
+else
+  refhash="$(git hash-object -t tree /dev/null)"
+fi
+
+random=$({ git var GIT_COMMITTER_IDENT ; echo "$refhash" ; cat "$1"; } | git hash-object --stdin)
 dest="$1.tmp.${random}"
 
-trap 'rm -f "${dest}"' EXIT
+trap 'rm -f "$dest" "$dest-2"' EXIT
 
 if ! git stripspace --strip-comments < "$1" > "${dest}" ; then
    echo "cannot strip comments from $1"
@@ -49,11 +61,40 @@ if test ! -s "${dest}" ; then
   exit 1
 fi
 
+reviewurl="$(git config --get gerrit.reviewUrl)"
+if test -n "${reviewurl}" ; then
+  token="Link"
+  value="${reviewurl%/}/id/I$random"
+  pattern=".*/id/I[0-9a-f]\{40\}$"
+else
+  token="Change-Id"
+  value="I$random"
+  pattern=".*"
+fi
+
+if git interpret-trailers --parse < "$1" | grep -q "^$token: $pattern$" ; then
+  exit 0
+fi
+
+# There must be a Signed-off-by trailer for the code below to work. Insert a
+# sentinel at the end to make sure there is one.
 # Avoid the --in-place option which only appeared in Git 2.8
-# Avoid the --if-exists option which only appeared in Git 2.15
-if ! git -c trailer.ifexists=doNothing interpret-trailers \
-      --trailer "Change-Id: I${random}" < "$1" > "${dest}" ; then
-  echo "cannot insert change-id line in $1"
+if ! git interpret-trailers \
+         --trailer "Signed-off-by: SENTINEL" < "$1" > "$dest-2" ; then
+  echo "cannot insert Signed-off-by sentinel line in $1"
+  exit 1
+fi
+
+# Make sure the trailer appears before any Signed-off-by trailers by inserting
+# it as if it was a Signed-off-by trailer and then use sed to remove the
+# Signed-off-by prefix and the Signed-off-by sentinel line.
+# Avoid the --in-place option which only appeared in Git 2.8
+# Avoid the --where option which only appeared in Git 2.15
+if ! git -c trailer.where=before interpret-trailers \
+         --trailer "Signed-off-by: $token: $value" < "$dest-2" |
+     sed -re "s/^Signed-off-by: ($token: )/\1/" \
+         -e "/^Signed-off-by: SENTINEL/d" > "$dest" ; then
+  echo "cannot insert $token line in $1"
   exit 1
 fi