Add script to clone entire GitHub organizations.

I found this to be most useful when I was trying to find things in the
LOCKSS codebase.

bin/download-github-repos

+#!/usr/bin/env bash
+#<
+# Download all public repositories for a given user or organization on GitHub.
+#
+# Usage: download-github-repos [<option>...] <who> -- <git arguments>
+#        download-github-repos -h
+#
+# Options:
+#   -d <outdir>         Save downloaded repositories under <outdir>.
+#   -f                  Force downloading into non-empty directory.
+#   -p <parallelism>    Run <parallelism> instances of git clone.
+#
+# When arguments to git clone are not provided, this passes --quiet.
+#
+# Commands:
+#   -h    Show this help.
+#
+# Example:
+#   download-github-repos lockss -- -q --recurse-submodules
+#>
+
+declare -r GITHUB_API=https://api.github.com
+declare -r EX_USAGE=64 EX_DATAERR=65
+
+fail() {
+    printf "%s\\n" "$2" >&2
+    exit $1
+}
+
+get_repo_urls() {
+    local org=$1 n page
+
+    # It seems that the "users" API includes both users and organizations;
+    # whereas the "orgs" API includes only the latter.
+    n=$(curl -s "$GITHUB_API/users/$org" | jq .public_repos)
+    [[ ! $n =~ ^[0-9]+$ ]] && exit 1
+
+    # Math is hard.
+    for ((page = 1; page <= (n + 99) / 100; page++)); do
+        curl -s "$GITHUB_API/users/$org/repos?page=$page&per_page=100" |
+            jq -r ".[].ssh_url"
+    done
+}
+
+set -euo pipefail
+shopt -s nullglob dotglob
+
+#
+# Process arguments.
+#
+
+declare force=0
+declare parallelism=4
+declare outdir=""
+declare org=""
+while getopts d:fp:h opt; do
+    case $opt in
+    d) outdir=$OPTARG ;;
+    f) force=1 ;;
+    p)
+        [[ $OPTARG =~ ^[0-9]+$ ]] || fail $EX_USAGE "Bad value for -p."
+        parallelism=$OPTARG ;;
+    h|*)
+        sed -ne '/^#</,/^#>/ { /^#\(<\|>\)/d; s/^# \?//; p; }' -- "$0"
+        [[ $opt == "?" ]] && exit $EX_USAGE
+        exit 0 ;;
+    esac
+done
+shift $((OPTIND - 1))
+
+[[ $# == 1 || $# -gt 1 && $2 == "--" ]] ||
+    fail $EX_USAGE "Wrong number of arguments."
+
+org=$1; shift
+[[ $org =~ ^[A-Za-z0-9_./-]+$ ]] || fail $EX_USAGE "Bad org name."
+
+: "${outdir:=$org}"
+if [[ ! -d $outdir ]]; then
+    mkdir -p -- "$outdir"
+elif ((!force)); then
+    if compgen -G "$outdir/*" >/dev/null; then
+        fail $EX_DATAERR \
+            "Output directory ${outdir@Q} is not empty. (Use -f to force.)"
+    fi
+fi
+
+if [[ ${1-} == "--" ]]; then
+    shift
+else
+    set -- --quiet
+fi
+
+type jq &>/dev/null || fail "Could not find required executable jq."
+
+#
+# Main.
+#
+
+cd -- "$outdir"
+get_repo_urls "$org" | xargs -r -I"{}" -P "$parallelism" git clone "{}" "$@"