fncache: avoid loading the filename cache when not actually modifying it With time, fncache can become very large. The mozilla-central repo for example, has a 31M and growing fncache file. Loading this file takes time (280ms for the mozilla-central repository). In many scenarios, we don't need to load fncache at all. For example, when committing changes to existing files, or pushing such commits to another clone. This patch detects when a name is added via store.vfs(), and only loads the cache if a) the data metadata file doesn't already exist, or b) when opening for appending, the data or metadata file exists but has size (a transaction rollback leaves behind such files). Benchmarks (run on Macos 10.13 on a 2017-model Macbook Pro with Core i7 2.9GHz and flash drive), each test without and with patch run 5 times: * committing to an existing file, against the mozilla-central repository. Baseline real time average 2.3736, with patch 1.9884. * unbundling a large changeset consisting *only* of existing-file modifications (159 revisions, 1050 modifications, mozilla-central 4a250a0e4f29:beea9ac7d823), into a clone limited to the ancestor revision of that revset). Baseline real time average 1.5048, with patch 1.3108.