revbranchcache: write cache even during read operations Previously we would only actually write the revbranchcache to disk if we were in the middle of a write operation (like commit). Now we will also write it during any read operation. The cache knows how to invalidate itself, so it shouldn't become corrupt if multiple writers try at once (and the write-on-read behavior/risk is the same as all our other caches).

--- a/tests/test-rebase-conflicts.t	Tue Feb 10 20:06:12 2015 -0800
+++ b/tests/test-rebase-conflicts.t	Tue Feb 24 18:43:31 2015 -0800
@@ -322,6 +322,6 @@
   files: 1/1 chunks (100.00%)
   added 2 changesets with 2 changes to 1 files
   invalid branchheads cache (served): tip differs
-  truncating cache/rbc-revs-v1 to 72
   rebase completed
   updating the branch cache
+  truncating cache/rbc-revs-v1 to 72