commit: move dirstateguard creation out of try-block

This is just a simple refactoring to make the next patch simpler. If
the dirstateguard constructor raises an exception, the finally-block
won't do anything anyway, so this is functionally equivalent (and
there is no except-block).

Differential Revision: https://phab.mercurial-scm.org/D405

mercurial/cmdutil.py

@@ -2998,9 +2998,10 b' def commit(ui, repo, commitfunc, pats, o'
     dsguard = None
     # extract addremove carefully -- this function can be called from a command
     # that doesn't support addremove
+    if opts.get('addremove'):
+        dsguard = dirstateguard.dirstateguard(repo, 'commit')
     try:
-        if opts.get('addremove'):
-            dsguard = dirstateguard.dirstateguard(repo, 'commit')
+        if dsguard:
             if scmutil.addremove(repo, matcher, "", opts) != 0:
                 raise error.Abort(
                     _("failed to mark all new/missing files as added/removed"))