Update bootstrap script from upstream gnulib to automatically fetch from repository if needed

See also: https://lists.gnu.org/archive/html/bug-gnulib/2020-08/msg00150.html

diff --git a/bootstrap b/bootstrap
index 8f76d6962..2d0a06376 100755
--- a/bootstrap
+++ b/bootstrap
@@ -670,6 +670,11 @@ if $use_gnulib; then
         || cleanup_gnulib
 
       trap - 1 2 13 15
+
+    elif test -n "$GNULIB_REVISION" \
+         && ! git --git-dir="$gnulib_path"/.git cat-file \
+              commit "$GNULIB_REVISION"; then
+      git --git-dir="$gnulib_path"/.git fetch
     fi
     GNULIB_SRCDIR=$gnulib_path
     ;;
@@ -704,6 +709,10 @@ if $use_gnulib; then
 
   if test -d "$GNULIB_SRCDIR"/.git && test -n "$GNULIB_REVISION" \
      && ! git_modules_config submodule.gnulib.url >/dev/null; then
+    if ! git --git-dir="$GNULIB_SRCDIR"/.git cat-file \
+         commit "$GNULIB_REVISION"; then
+      git --git-dir="$GNULIB_SRCDIR"/.git fetch
+    fi
     (cd "$GNULIB_SRCDIR" && git checkout "$GNULIB_REVISION") || cleanup_gnulib
   fi