rebase: use one dirstateguard for when using rebase.singletransaction This was previously landed as 2519994d25ca but backed out in b63351f6a2 because it broke hooks mid-rebase and caused conflict resolution data loss in the event of unexpected exceptions. This new version adds the behavior back but behind a config flag, since the performance improvement is notable in large repositories. The old commit message was: Recently we switched rebases to run the entire rebase inside a single transaction, which dramatically improved the speed of rebases in repos with large working copies. Let's also move the dirstate into a single dirstateguard to get the same benefits. This let's us avoid serializing the dirstate after each commit. In a large repo, rebasing 27 commits is sped up by about 20%. I believe the test changes are because us touching the dirstate gave the transaction something to actually rollback. (grafted from 9e3dc3a1638b9754b58a0cb26aaa75d868058109) (grafted from 7d38b41d2266d9a02a15c64229fae0da5738dcec) Differential Revision: https://phab.mercurial-scm.org/D135

--- a/tests/test-rebase-base.t	Tue Aug 01 10:14:25 2017 -0400
+++ b/tests/test-rebase-base.t	Thu Jul 20 01:30:41 2017 -0700
@@ -379,3 +379,40 @@
    /
   o  0: A
   
+Rebasing using a single transaction
+
+  $ hg init singletr && cd singletr
+  $ cat >> .hg/hgrc <<EOF
+  > [rebase]
+  > singletransaction=True
+  > EOF
+  $ hg debugdrawdag <<'EOF'
+  >   Z
+  >   |
+  >   | D
+  >   | |
+  >   | C
+  >   | |
+  >   Y B
+  >   |/
+  >   A
+  > EOF
+- We should only see two status stored messages. One from the start, one from
+- the end.
+  $ hg rebase --debug -b D -d Z | grep 'status stored'
+  rebase status stored
+  rebase status stored
+  $ hg tglog
+  o  5: D
+  |
+  o  4: C
+  |
+  o  3: B
+  |
+  o  2: Z
+  |
+  o  1: Y
+  |
+  o  0: A
+  
+  $ cd ..