rebase: use single transaction when running in memory

rebase.singletransaction make rebase noticeably faster (~20% in a test
I just ran). It is not enabled by default because it risks losing
information if it aborts (see `hg help rebase`). When running rebase
with the experimental in-memory option on, rebase is first attempted
in memory, and if any conflicts occur, it restarts, this time writing
to disk. Thus, it should be safe to turn on single-transaction mode
for the in-memory phase.

Differential Revision: https://phab.mercurial-scm.org/D3076

hgext/rebase.py

@@ -788,6 +788,8 b' def rebase(ui, repo, **opts):'
         try:
             # in-memory merge doesn't support conflicts, so if we hit any, abort
             # and re-run as an on-disk merge.
+            overrides = {('rebase', 'singletransaction'): True}
+            with ui.configoverride(overrides, 'rebase'):
             return _origrebase(ui, repo, inmemory=inmemory, **opts)
         except error.InMemoryMergeConflictsError:
             ui.warn(_('hit merge conflicts; re-running rebase without in-memory'