R1:3edf60627d5b

Add support for marking files as "generated" by regexp against path

Summary:
Not all auto-generated files can include the magical
"generated" annotation for one reason or another, but they may follow
path rules. This patch allows files to be marked as automatically
generated by matching the path with a regular expression.

Test Plan:
Alter 'differential.generated-paths' setting in config.
Create a new diff that affects a file matching one of those regular
expressions. Verify that Differential marks it as…