tests: simplify test-rebase-transaction.t

The file was extracted from test-rebase-base.t in 8cef8f7d51d0
(test-rebase-base: clarify it is about the "--base" flag,
2017-10-05). This patch follows up that and clarifies the new file's
purpose and simplifies it a bit.

Differential Revision: https://phab.mercurial-scm.org/D2756

tests/test-rebase-transaction.t

@@ -1,49 +1,50 b''
+Rebasing using a single transaction
+
   $ cat >> $HGRCPATH <<EOF
   > [extensions]
   > rebase=
   > drawdag=$TESTDIR/drawdag.py
   > 
+  > [rebase]
+  > singletransaction=True
+  > 
   > [phases]
   > publish=False
   > 
   > [alias]
   > tglog = log -G --template "{rev}: {desc}"
   > EOF
 
-Rebasing using a single transaction
+Check that a simple rebase works
 
-  $ hg init singletr && cd singletr
-  $ cat >> .hg/hgrc <<EOF
-  > [rebase]
-  > singletransaction=True
-  > EOF
+  $ hg init simple && cd simple
   $ 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 ..