rebase: handle the case when nothing to rebase (dry-run)

In dry-run mode, even when there was no rebase in progress it tried
to abort (for context, at the end of dry-run mode we abort the in-memory
performed rebase) because we were not early checking if there was any rebase
started.

Changes in test file reflect the fixed behavior.

Differential Revision: https://phab.mercurial-scm.org/D9659

hgext/rebase.py

@@ -1139,13 +1139,16 b' def _dryrunrebase(ui, repo, action, opts'
         try:
             overrides = {(b'rebase', b'singletransaction'): True}
             with ui.configoverride(overrides, b'rebase'):
-                _origrebase(
+                res = _origrebase(
                     ui,
                     repo,
                     action,
                     opts,
                     rbsrt,
                 )
+                if res == _nothingtorebase():
+                    needsabort = False
+                    return res
         except error.ConflictResolutionRequired:
             ui.status(_(b'hit a merge conflict\n'))
             return 1

tests/test-rebase-inmemory.t

@@ -342,9 +342,7 b' When nothing to rebase'
   $ hg reb -r . -d '.^' -n
   starting dry-run rebase; repository will not be changed
   nothing to rebase
-  dry-run rebase completed successfully; run without -n/--dry-run to perform this rebase
-  abort: no rebase in progress
-  [20]
+  [1]
 
 Check dryrun gives correct results when there is no conflict in rebasing
   $ hg rebase -s 2 -d 6 -n