Saving with only a filename (from Python) wasn't being prevented,
while it would successfully write the file to the working-directory,
path remapping and setting relative paths wouldn't work afterwards
as `Main.filepath` would have no directory component.
Disallow this since it's a corner case which only ever occurs
when path names without any directories are used from Python,
the overhead of expanding the working-directory for all data saving
operations isn't worthwhile.
The following changes have been made:
- bpy.ops.wm.save_mainfile() without a filepath argument
fails & reports and error when the file hasn't been saved.
Previously it would write to "untitled.blend" and set the
`G.main->filepath` to this as well.
- bpy.ops.wm.save_mainfile(filepath="untitled.blend")
fails & reports and error as the filename has no directory component.
- `BLI_path_is_abs_from_cwd` was added to check if the path would
attempt to expand to the CWD.
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