Improve Pootle FS sync to disk

[unho]

Syncing to disk with Pootle FS introduces more changes than desired:

• PO header comments are gone (example)
• PO header fields are not updated correctly (example)
  • Pootle POT header is updated instead of PO header line
  • Last-Translator is reset and not updated
  • PO header is set to an unset header
• Introduced extra PO header fields (example)
• Obsolete units are removed from file (example)
• Untouched units in Pootle are reflown in file (example)

All of these don't happen when using sync_stores, but do happen with Pootle FS either using localfs or git plugin.

The expected result is that each sync introduces the minimum necessary diff, to reduce the noise.

[phlax]

pootle fs now uses the same code path as store.sync does and i believe these issues have been addressed