Emacsg: add bin/borg-bootstrap script

See the commentary at the beginning of the script for usage
instructions.  Starting with the next commit `make bootstrap'
will use it.

bin/borg-bootstrap

@@ -0,0 +1,97 @@
+#!/bin/sh
+
+# Copyright (C) 2016  Jonas Bernoulli
+#
+# Author: Jonas Bernoulli <jonas@bernoul.li>
+# License: GPL v3 <https://www.gnu.org/licenses/gpl-3.0.txt>
+
+# Commentary:
+
+# This script is a replacement for `git submodule update'.  The
+# main differences are that this script resets the current branch
+# if necessary instead of detaching HEAD and that it is capable of
+# adding and configuring additional remotes.
+
+# The following variables can be set in ".gitmodules":
+#
+# * submodule.DRONE.remote = NAME URL
+#
+#   This variable specifies an additional remote named DRONE that is
+#   fetched from URL.
+#
+# * borg.collective = REMOTE
+#
+#   This variable specifies the name used for remotes that reference
+#   a repository that has been patched by the collective.  If a NAME
+#   matches REMOTE, then it is configured as the upstream of the
+#   current branch of the respective DRONE.
+#
+# * borg.pushDefault = DEFAULT
+#
+#   This variable specifies a name used for push-remotes.  If a NAME
+#   matches DEFAULT, then it is configured as the push-remote of the
+#   current branch of the respective DRONE.
+
+# If an error occurs while `make bootstrap' runs this script, then
+# fix the error(s) and run `bin/borg-bootstrap' directly, followed
+# by `make build'.
+
+hive_remote=$(git config -f .gitmodules borg.collective)
+push_remote=$(git config -f .gitmodules borg.pushDefault)
+
+git submodule--helper list |
+while read mode sha1 stage path
+do
+    name=$(git submodule--helper name "$path")
+    url=$(git config -f .gitmodules submodule."$name".url)
+
+    if test -e "$path" && ! test -e "$path"/.git
+    then
+        git submodule--helper clone --name "$name" --path "$path" --url "$url"
+
+        git config -f .gitmodules --get-all submodule."$name".remote |
+        while read remote remote_url
+        do
+            if ! test -e "$path"/.git
+            then
+                git submodule--helper clone \
+                    --name "$name" \
+                    --path "$path" \
+                    --url "$remote_url" &&
+                git remote rename origin "$remote"
+            else
+                (
+                    cd "$path"
+                    git remote add "$remote" "$remote_url"
+                    git fetch "$remote"
+                )
+            fi
+
+            if test -e "$path"/.git
+            then
+                (
+                    cd "$path"
+                    if test "$remote" = "$hive_remote"
+                    then
+                        git config branch.master.remote "$remote"
+
+                    elif test "$remote" = "$push_remote"
+                    then
+                        git config remote.pushDefault "$remote"
+                    fi
+                )
+            fi
+        done
+
+        if test -e "$path"/.git
+        then
+            (
+                cd "$path"
+                git reset --hard "$sha1" ||
+                echo >&2 "futile: checkout of '$sha1' into submodule path '$path' failed"
+            )
+        else
+            echo >&2 "futile: clone of any remote into submodule path '$path' failed"
+        fi
+    fi
+done