convert/svn: update svn working copy only when necessary

I have not tried to produce the bug but here is idea: f85c0034a062 stopped
passing the modified files list to commit. This makes commit more fragile since
we better not touch unrelated files by mistake. But putcommit() still applies
file changes before exiting upon ignored revisions. So in theory, we could
apply changes from a skipped branch then commit them as part of another
revision.

This patch makes the sink apply the changes after possibly skipping the
revision. The real fix would be to use svn commit --targets option to pass the
file names in an argument file. Unfortunately, it seems to be bugged in svn
1.7.1:

  http://svn.haxx.se/dev/archive-2011-11/0211.shtml

hgext/convert/subversion.py

@@ -1116,6 +1116,12 b' class svn_sink(converter_sink, commandli'
         return u"svn:%s@%s" % (self.uuid, rev)
 
     def putcommit(self, files, copies, parents, commit, source, revmap):
+        for parent in parents:
+            try:
+                return self.revid(self.childmap[parent])
+            except KeyError:
+                pass
+
         # Apply changes to working copy
         for f, v in files:
             try:
@@ -1128,11 +1134,6 b' class svn_sink(converter_sink, commandli'
                     self.copies.append([copies[f], f])
         files = [f[0] for f in files]
 
-        for parent in parents:
-            try:
-                return self.revid(self.childmap[parent])
-            except KeyError:
-                pass
         entries = set(self.delete)
         files = frozenset(files)
         entries.update(self.add_dirs(files.difference(entries)))