etc: Update the commit-msg hook.

Update our copy to the latest version retrieved from
<https://gerrit.googlesource.com/gerrit/+/refs/heads/master/resources/com/google/gerrit/server/tools/root/hooks/commit-msg>.

This change contains an improved version which avoids creating duplicate
Change-Id git trailer when rebasing.

* etc/git/commit-msg: Update to latest version, taking care to preserve our changes.

Change-Id: Ie6ae6aa5e81cd4fce28a6be5cd68ca0a61cdebc0

etc/git/commit-msg

@@ -1,5 +1,5 @@
 #!/bin/sh
-# From Gerrit Code Review 3.6.1
+# From Gerrit Code Review 3.11.1.
 #
 # Part of Gerrit Code Review (https://www.gerritcodereview.com/)
 #
@@ -44,9 +44,20 @@ if test ! -f "$1" ; then
 fi
 
 # Do not create a change id if requested
-if test "false" = "$(git config --bool --get gerrit.createChangeId)" ; then
-  exit 0
-fi
+case "$(git config --get gerrit.createChangeId)" in
+  false)
+    exit 0
+    ;;
+  always)
+    ;;
+  *)
+    # Do not create a change id for squash/fixup commits.
+    if head -n1 "$1" | LC_ALL=C grep -q '^[a-z][a-z]*! '; then
+      exit 0
+    fi
+    ;;
+esac
+
 
 if git rev-parse --verify HEAD >/dev/null 2>&1; then
   refhash="$(git rev-parse HEAD)"
@@ -57,9 +68,9 @@ fi
 random=$({ 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
+if ! cat "$1" | sed -e '/>8/q' | git stripspace --strip-comments > "${dest}" ; then
    echo "cannot strip comments from $1"
    exit 1
 fi
@@ -71,21 +82,39 @@ fi
 
 reviewurl="$(git config --get gerrit.reviewUrl)"
 if test -n "${reviewurl}" ; then
-  if ! git interpret-trailers --parse < "$1" | grep -q '^Link:.*/id/I[0-9a-f]\{40\}$' ; then
-    if ! git interpret-trailers \
-          --trailer "Link: ${reviewurl%/}/id/I${random}" < "$1" > "${dest}" ; then
-      echo "cannot insert link footer in $1"
-      exit 1
-    fi
-  fi
+  token="Link"
+  value="${reviewurl%/}/id/I$random"
+  pattern=".*/id/I[0-9a-f]\{40\}"
 else
-  # Avoid the --in-place option which only appeared in Git 2.8
-  # Avoid the --if-exists option which only appeared in Git 2.15
-  if ! git -c trailer.ifexists=doNothing interpret-trailers \
-        --trailer "Change-Id: I${random}" < "$1" > "${dest}" ; then
-    echo "cannot insert change-id line in $1"
-    exit 1
-  fi
+  token="Change-Id"
+  value="I$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
+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 -e "s/^Signed-off-by: \($token: \)/\1/" \
+         -e "/^Signed-off-by: SENTINEL/d" > "$dest" ; then
+  echo "cannot insert $token line in $1"
+  exit 1
 fi
 
 if ! mv "${dest}" "$1" ; then