rebase: also include commit of collapsed commits in single transaction

Authored by martinvonz.

Description

rebase: also include commit of collapsed commits in single transaction

When rebase.singletransaction is set, we still used to create a second
transaction when committing with --collapse. It's simpler to create a
single transaction.

Note that in the affected .t file, the test that uses --collapse still
appears to create two transactions (it prints "rebase status stored"
twice). However, only a single transaction is actually created and the
second printout comes from cmdutil.commitforceeditor() that explicitly
calls tr.writepending().

Also note the that we now roll back any commits if the user closes the
commit message editor with an error code (or leaves the message
empty). That might be unfortunate, but it's consistent with how we
behave in the --no-collapse case (if the user passed --edit). If we
want to change that, I think it should be done consistently in a
separate patch.

Differential Revision: https://phab.mercurial-scm.org/D2728