patchf4b04cf65 ("git-subtree: I suggest to add following lines into commit message", 2018-05-31)

contrib/subtree/git-subtree.sh, contrib/subtree/git-subtree.txt

diff --git a/contrib/subtree/git-subtree.sh 
b/contrib/subtree/git-subtree.sh
index d3f39a862..969649530 100755
--- a/contrib/subtree/git-subtree.sh
+++ b/contrib/subtree/git-subtree.sh
@@ -15,8 +15,10 @@ git subtree merge --prefix=<prefix> <commit>
  git subtree pull  --prefix=<prefix> <repository> <ref>
  git subtree push  --prefix=<prefix> <repository> <ref>
  git subtree split --prefix=<prefix> <commit...>
+git subtree --list
  --
  h,help        show the help
+l,list        show externals
  q             quiet
  d             show debug messages
  P,prefix=     the name of the subdir to split out
@@ -48,6 +50,9 @@ annotate=
  squash=
  message=
  prefix=
+repo=
+ref=
+list=

  debug () {
  	if test -n "$debug"
@@ -77,6 +82,119 @@ assert () {
  	fi
  }

+show_externals () {
+  debug "Looking for externals..."
+  dir=
+  rev=
+  updated=
+  commit=
+  repo=
+  ref=
+  git log --grep="^git-subtree-dir: .*\$" \
+          --no-show-signature --pretty=format:'START 
%H%n%s%n%n%b%nEND%n' |
+  while read a b junk
+  do
+    case "$a" in
+    START)
+      commit="$b"
+      ;;
+    git-subtree-dir:)
+      if test -n "$dir"
+      then
+        if test "$dir" = "$b"
+        then
+          break
+        fi
+      fi
+      dir="$b"
+      ;;
+    git-subtree-split:)
+      rev="$b"
+      ;;
+    git-subtree-repo:)
+      repo="$b"
+      ;;
+    git-subtree-ref:)
+      ref="$b"
+      ;;
+    END)
+      if test -n "$dir" -a -e "$dir"
+      then
+        debug ""
+        debug "Commit: $commit"
+        if test -n "$repo"
+        then
+          output="$(git ls-remote $repo)"
+          if test -n "$ref"
+          then
+            if $(echo "$output" | grep -q $ref)
+            then
+              hash="$(echo "$output" | grep "$ref\$" | cut -f1 
-d$'\t')"
+              if test "$hash" != "$rev"
+              then
+                updated="$rev"
+                rev="$hash"
+              fi
+            fi
+          fi
+
+          output="$(echo "$output" | grep $rev)"
+          if test -n "$output"
+          then
+            echo "$output" |
+            while read h r junk
+            do
+              case "$r" in
+                HEAD)
+                  head="yes"
+                  ;;
+                refs/heads/*)
+                  name=$(basename $r)
+                  type="branch"
+                  echo -n "$dir $repo $type $name"
+                  if test "$head" = "yes"
+                  then
+                    echo " HEAD"
+                  else
+                    echo " $rev"
+                  fi
+                  break
+                  ;;
+                refs/tags/*)
+                  name=$(basename $r)
+                  type="tag"
+                  echo "$dir $repo $type $name $rev"
+                  break
+                  ;;
+              esac
+            done
+            if test -n "$updated"
+            then
+              debug ""
+              debug "The '$dir' subtree seems not updated:"
+              debug "   original revision: $updated"
+              debug "    reemote revision: $rev"
+              debug "You can update '$dir' subtree by following 
command:"
+              debug ""
+              debug "   git subtree pull --prefix=$dir $repo $ref"
+              debug ""
+            fi
+          fi
+        else
+          echo "$dir $rev"
+        fi
+      fi
+
+      rev=
+      updated=
+      commit=
+      repo=
+      ref=
+      ;;
+    esac
+  done
+}
+

  while test $# -gt 0
  do
@@ -137,6 +255,9 @@ do
  	--no-squash)
  		squash=
  		;;
+	-l)
+		list=yes
+		;;
  	--)
  		break
  		;;
@@ -149,6 +270,12 @@ done
  command="$1"
  shift

+if test "$list" = "yes"
+then
+  show_externals
+  exit 0
+fi
+
  case "$command" in
  add|merge|pull)
  	default=
@@ -421,6 +548,18 @@ add_msg () {
  		git-subtree-mainline: $latest_old
  		git-subtree-split: $latest_new
  	EOF
+	if test -n "$repo"
+	then
+		cat <<-EOF
+			git-subtree-repo: $repo
+		EOF
+	fi
+	if test -n "$ref"
+	then
+		cat <<-EOF
+			git-subtree-ref: $ref
+		EOF
+	fi
  }

  add_squashed_msg () {
@@ -622,6 +761,8 @@ cmd_add () {
  		# branches fetched via the refspec.
  		ensure_valid_ref_format "$2"

+		repo=$1
+		ref=$2
  		cmd_add_repository "$@"
  	else
  		say "error: parameters were '$@'"
diff --git a/contrib/subtree/git-subtree.txt 
b/contrib/subtree/git-subtree.txt
index 352deda69..4927f9767 100644
--- a/contrib/subtree/git-subtree.txt
+++ b/contrib/subtree/git-subtree.txt
@@ -15,6 +15,7 @@ SYNOPSIS
  'git subtree' push  -P <prefix> <repository> <ref>
  'git subtree' merge -P <prefix> <commit>
  'git subtree' split -P <prefix> [OPTIONS] [<commit>]
+'git subtree' --list


  DESCRIPTION
@@ -131,6 +132,10 @@ OPTIONS
  --debug::
  	Produce even more unnecessary output messages on stderr.

+-l::
+--list::
+	Print the list of joined subtrees and exit.
+
  -P <prefix>::
  --prefix=<prefix>::
  	Specify the path in the repository to the subtree you