Only run sha1_file once per file in FSFile

This shouldn't harm as we don't use this and then alter the file.
But it can kick in useful with extension that need the sha1 of
files (like AbuseFilter).

Change-Id: Ied2ac0397266a5ffd27da02ef6448b64c12fad86
Merge "Only run sha1_file once per file in FSFile"