Fix: MercurialHandler does read the version number properly

MercurialHandler fails to retrieve the version number of a working copy if the last changeset did not affect the root folder "csb" (e.g. after adding a tag).

csb/build.py

@@ -538,23 +538,30 @@ def __init__(self, path, sc='hg'):
         super(MercurialHandler, self).__init__(path, sc)
 
     def read(self):
-
-        cmd = '{0.sc} log -r tip {0.path}'.format(self)
-
-        revision = None
-        changeset = ''
 
-        for line in self._run(cmd):
-            if line.startswith('changeset:'):
-                items = line[10:].split(':')
-                revision = int(items[0])
-                changeset = items[1].strip()
-                break
-
-        if revision is None:
-            raise RevisionError('No revision number found', code=0, cmd=cmd)
+        wd = os.getcwd()
+        os.chdir(self.path)
+
+        try:
+            cmd = '{0.sc} log -r tip'.format(self)
+
+            revision = None
+            changeset = ''
+
+            for line in self._run(cmd):
+                if line.startswith('changeset:'):
+                    items = line[10:].split(':')
+                    revision = int(items[0])
+                    changeset = items[1].strip()
+                    break
+
+            if revision is None:
+                raise RevisionError('No revision number found', code=0, cmd=cmd)
+
+            return RevisionInfo(self.path, revision, changeset)  
 
-        return RevisionInfo(self.path, revision, changeset)  
+        finally:
+            os.chdir(wd)
 
 class RevisionInfo(object):