phabricator: avoid calling differential.getcommitmessage Previously, we call differential.getcommitmessage API to get commit messages. Now we read that from "Differential Revision" object fetched via "differential.query" API. This removes one API call per patch.

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.

phabricator: add node and p1 to hg:meta property The "hg:meta" property is for extra metadata to reconstruct the patch. Previously it does not have node or parent information since I think by reading the patch again, the commit message will be mangled (like, added the "Differential Revision" line) and we cannot preserve the commit hash. However, the "parent" information could be useful. It could be helpful to locate the "base revision" so in case of a conflict applying the patch directly, we might be able to use 3-way merge to resolve it correctly. Note: "local:commits" is an existing "property" used by Phabricator that has the node and parent information. However, it lacks of timezone information and requires "author" and "authorEmail" to be separated. So we are using a different "property" - "hg:meta" to be distinguished from "local:commits".

phabricator: check associated Differential Revision from commit message Previously, only tags can "associate" a changeset to a Differential Revision. But the usual pattern (arc patch or hg phabread) is to put the Differential Revision URL in commit message. This patch makes the code read commit message to find associated Differential Revision if associated tags are not found. This makes some workflows possible. For example, if the author loses their repo, or switch to another computer, they can continue download their own patches from Phabricator and update them without needing to manually create tags.

tests: replace yet more calls to `python` with $PYTHON These are some simple cases. More to come in a future change. Reviewers: krbullock Reviewed By: krbullock Differential Revision: https://phab.mercurial-scm.org/D4

tests: capitalize Python when it's not used as a command name This avoids some false positives in an upcoming check-code rule. Reviewers: krbullock Reviewed By: krbullock Differential Revision: https://phab.mercurial-scm.org/D3