tests: actually test non-generaldelta variant for stream clones 608cabec1b15 accidentally made both tests test generaldelta repos. Restore the test for a non-gd repo.

revlog: document high frequency of code execution Recording my notes while working on performance optimization.

revlog: make code in builddelta() slightly easier to read self.compress() is destructured into its components. "l" is renamed to "deltalen."