reproduce.tt: Set revCount for hg repos

Dependencies

• [2] HZWUT4YN Allow users to reproduce builds on their own systems
• [3] 5Q4ZLL37 reproduce.tt: Add --help flag
• [4] 3G7OYUSE reproduce.tt: Check for the presence of commands like nix-build

Change contents

• replacement in src/root/reproduce.tt at line 113
  [2.1931]→[2.1931:2003](∅→∅)
  − hg clone '[% input.uri %]' "$inputDirTmp" -r '[% input.revision %]'
  [2.1931]

  [2.2003]
  + hg clone '[% input.uri %]' "$inputDirTmp"

  + (cd "$inputDirTmp" && hg update '[% input.revision %]')

  + revCount="$(cd "$inputDirTmp" && (hg log -r '[% input.revision %]' --template "{rev}"))"