make roll_forward and roll_back more symmetric

Dependencies

• [2] KGILP75E remove abstraction: update_head
• [3] 5T6SHEIT abstraction: roll forward/back
• [4] JUTECG5X bring template-live's protocol in sync with template-live-editor
• [5] OI2U4SQ3 extract a function
• [6] T3LA4DVG new command: batch GET
• [7] TNRO6KLZ new live app
• [8] OJLXS67N a prefix for metadata in the manifest
• [9] UQLKUPWR start including a default_map with each app
• [10] 5TZBSRBN audit all calls to app.eval
• [*] JIWWTY7R extract a function

Change contents

• edit in main.lua at line 241
  [3.6269]→[3.63:99](∅→∅),[3.99]→[3.6304:6447](∅→∅),[3.6304]→[3.6304:6447](∅→∅)
  − app.Manifest[PARENT] = app.Head

  − local manifest_filename = app.versioned_manifest(app.Next_version)

  − love.filesystem.write(manifest_filename, json.encode(app.Manifest))
• edit in main.lua at line 266
  [3.6954]→[3.100:136](∅→∅),[3.136]→[3.6989:7132](∅→∅),[3.6989]→[3.6989:7132](∅→∅)
  − app.Manifest[PARENT] = app.Head

  − local manifest_filename = app.versioned_manifest(app.Next_version)

  − love.filesystem.write(manifest_filename, json.encode(app.Manifest))
• edit in main.lua at line 278
  [12.32]

  [3.52]
  + -- update app.Head and record the new app.Manifest (which caller has already modified)
• edit in main.lua at line 280
  [3.80]

  [2.4]
  + app.Manifest[PARENT] = app.Head

  + local manifest_filename = app.versioned_manifest(app.Next_version)

  + love.filesystem.write(manifest_filename, json.encode(app.Manifest))
• edit in main.lua at line 288
  [3.163]

  [12.32]
  + -- update app.Head and reload app.Manifest appropriately