Add AMPB browser source build spike tools

tools/browser-source-sync.sh

@@ -0,0 +1,40 @@
+#!/bin/sh
+set -eu
+
+root="${AMPB_BROWSER_BUILD_ROOT:-/tmp/ampb-browser-build}"
+src="$root/src"
+
+gecko_repo="${AMPB_GECKO_REPO:-https://github.com/mozilla/gecko-dev.git}"
+legacy_android_repo="${AMPB_LEGACY_FIREFOX_ANDROID_REPO:-https://github.com/mozilla-mobile/firefox-android.git}"
+tor_build_repo="${AMPB_TOR_BROWSER_BUILD_REPO:-https://gitlab.torproject.org/tpo/applications/tor-browser-build.git}"
+
+mkdir -p "$src"
+
+sync_repo() {
+  repo_url="$1"
+  dest="$2"
+  mode="$3"
+  if [ -d "$dest/.git" ]; then
+    if [ "$mode" = "blobless" ]; then
+      git -C "$dest" fetch --depth 1 --filter=blob:none origin
+    else
+      git -C "$dest" fetch --depth 1 origin
+    fi
+    git -C "$dest" reset --hard FETCH_HEAD
+  else
+    if [ "$mode" = "blobless" ]; then
+      git clone --depth 1 --filter=blob:none "$repo_url" "$dest"
+    else
+      git clone --depth 1 "$repo_url" "$dest"
+    fi
+  fi
+}
+
+sync_repo "$gecko_repo" "$src/gecko-dev" "blobless"
+sync_repo "$tor_build_repo" "$src/tor-browser-build" "normal"
+
+if [ "${AMPB_INCLUDE_LEGACY_FIREFOX_ANDROID:-0}" = "1" ]; then
+  sync_repo "$legacy_android_repo" "$src/firefox-android" "normal"
+fi
+
+printf 'AMPB_SOURCE_SYNC status=ok root=%s\n' "$root"