Blender Git Loki

Prevent G.fileflags changes when WM_OT_save_mainfile() is called from script

This is to solve an issue where a blend file could be compressed
unbeknownst to the artist. This happened in the following situtation:

- Artist edits an uncompressed blend file.
- Some script saves a compressed blendfile to a separate location.
- When the artist saves the file (s)he is editing (File>Save, or Ctrl+S),
it was silently compressed.

Cherry pick from: cd3b313d5f44a10a1150bf1ddb560775d1bcd827