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.

phabricator: abort if phabsend gets empty revs Previously we didn't abort. Now we abort if revs is empty. This is consistent with "hg export" behavior. Maybe "return 1" is also a reasonable behavior, but that's inconsistent with the existing "hg export".

phabricator: do not upload new diff if nothing changes Previously, `phabsend` uploads new diffs as long as the commit hash changes. That's suboptimal because sometimes the diff is exactly the same as before, the commit hash change is caused by a parent hash change, or commit message change which do not affect diff content. This patch adds a check examining actual diff contents to skip uploading new diffs in that case.