templatefilters: add new stripdir filter

Adds a new template filter for removing directory levels from a
string. Examples:

{foo|stripdir} -> 'foo'
{foo/bar|stripdir} -> 'foo'
{foo/bar/more|stripdir} -> 'foo/bar'
{foo/bar/more|stripdir|stripdir} -> 'foo'

mercurial/templatefilters.py

@@ -157,9 +157,18 b' def json(obj):'
     else:
         raise TypeError('cannot encode type %s' % obj.__class__.__name__)
 
+def stripdir(text):
+    '''Treat the text as path and strip a directory level, if possible.'''
+    dir = os.path.dirname(text)
+    if dir == "":
+        return os.path.basename(text)
+    else:
+        return dir
+
 filters = {
     "addbreaks": nl2br,
     "basename": os.path.basename,
+    "stripdir": stripdir,
     "age": age,
     "date": lambda x: util.datestr(x),
     "domain": domain,