maintainers/scripts/rebuild-amount.sh: report parallelism, add example, cleanup (#137695)

* maintainers/scripts/rebuild-amount.sh: report parallelism setting

This slightly helps discovering parallelism support.

* maintainers/scripts/rebuild-amount.sh: add basic example

It's not clear if script should be used against HEAD or HEAD^.
Let's have a copyable example.

* maintainers/scripts/rebuild-amount.sh: convert tabs to four spaces

One notable change is here-document conversion.
Before:

  <TAB>cat <<-FOO
  <TAB>...
  <TAB>FOO

After:

  <sp><sp><sp><sp>cat <<FOO
  <sp><sp><sp><sp>...
  FOO

Note seemingly misaligned 'FOO': '-FOO' understands leading tabs,
but not spaces.

maintainers/scripts/rebuild-amount.sh

@@ -9,10 +9,17 @@ if [ "$1" == "--print" ]; then
 fi
 
 if [ "$#" != 1 ] && [ "$#" != 2 ]; then
-	cat <<-EOF
-	Usage: $0 [--print] commit-spec [commit-spec]
+    cat <<EOF
+    Usage: $0 [--print] from-commit-spec [to-commit-spec]
         You need to be in a git-controlled nixpkgs tree.
         The current state of the tree will be used if the second commit is missing.
+
+        Examples:
+          effect of latest commit:
+              $ $0 HEAD^
+              $ $0 --print HEAD^
+          effect of the whole patch series for 'staging' branch:
+              $ $0 origin/staging staging
 EOF
     exit 1
 fi
@@ -21,7 +28,7 @@ fi
 parallel="$(echo 'config.rebuild-amount.parallel or false' | nix-repl . 2>/dev/null \
             | grep -v '^\(nix-repl.*\)\?$' | tail -n 1 || true)"
 
-echo "Estimating rebuild amount by counting changed Hydra jobs."
+echo "Estimating rebuild amount by counting changed Hydra jobs (parallel=${parallel:-unset})."
 
 toRemove=()
 
@@ -33,7 +40,7 @@ trap cleanup EXIT SIGINT SIGQUIT ERR
 MKTEMP='mktemp --tmpdir nix-rebuild-amount-XXXXXXXX'
 
 nixexpr() {
-	cat <<-EONIX
+    cat <<EONIX
         let
           lib = import $1/lib;
           hydraJobs = import $1/pkgs/top-level/release.nix