test: introduce a new flag to display env variable line per line It's easier to conditionalize some of the environment variables per Mercurial version once there is only one value per line. Differential Revision: https://phab.mercurial-scm.org/D5453

revlog: add an explicit test for `issnapshot` We test the method on a real revlog containing "real" data.

revlog: add some direct testing of the slicing logic This test check slicing backed by an actual revlog. It will test the C version of slicing (if the test are run with the C extensions).

revlog: limit base to rev size ratio to 500 instead of 50 While a value of 50 provided large speedup in some case (400s → 7s) it also creates a slow down for a whole class of revision we are seeing in a private repository (0.1s → 3s). A value of 500 makes them disappear improving the total runtime (the slower revision still improve significantly (400s → 21s)). We need to run a wider array of tests on various repositories to see the effect on speed and size of different values for this acceptable constant. However, in the meantime, it seems safer to move back to a less impactful value.