Make cd() overwrite cwd when given an absolute path.

Fixes fabric#166.

docs/changes/0.9.1.rst

@@ -25,6 +25,9 @@ Bugfixes
 * :issue:`132`: `~fabric.operations.local` now calls ``strip`` on its stdout,
   matching the behavior of `~fabric.operations.run`/`~fabric.operations.sudo`.
   Thanks to Carl Meyer again on this one.
+* :issue:`166`: `~fabric.context_managers.cd` now correctly overwrites
+  ``env.cwd`` when given an absolute path, instead of naively appending its
+  argument to ``env.cwd``'s previous value.
 
 
 Documentation updates

fabric/context_managers.py

@@ -181,7 +181,7 @@ def cd(path):
         the *behavior* of `cd` will have any guarantee of backwards
         compatibility.
     """
-    if env.get('cwd'):
+    if env.get('cwd') and not path.startswith('/'):
         new_cwd = env.cwd + '/' + path
     else:
         new_cwd = path

tests/test_context_managers.py

@@ -24,3 +24,17 @@ class TestException(Exception):
     finally:
         with cd('else'):
             eq_(env.cwd, 'else')
+
+
+def test_cwd_with_absolute_paths():
+    """
+    cd() should append arg if non-absolute or overwrite otherwise
+    """
+    existing = '/some/existing/path' 
+    additional = 'another'
+    absolute = '/absolute/path'
+    env.cwd = existing
+    with cd(absolute):
+        eq_(env.cwd, absolute)
+    with cd(additional):
+        eq_(env.cwd, existing + '/' + additional)