phabricator: rework phabread to reduce memory usage and round-trips This patch reworked phabread a bit so it fetches the lightweight "Differential Revision" metadata for a stack first. Then read other data. This allows the code to: a) send 1 request to get all `hg:meta` data instead of N requests b) patches are read in desired order, no need to buffer the output "b)" reduces the memory usage from O(N^2) to O(N) since we no longer keep old patch contents in memory. The above `N` means the number of patches in the stack.