Specify merge tool for hg rebase

Otherwise we might fall back into editor, what is not something we want
to execute.

Issue #684

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/vcs.py

@@ -739,7 +739,7 @@ def rebase(self, branch=None, abort=False):
             self.execute(['rebase', '--abort'])
         else:
             try:
-                self.execute(['rebase'])
+                self.execute(['rebase', '--tool', 'internal:merge'])
             except RepositoryException as error:
                 if error.retcode == 1:
                     self.execute(['update'])