Hg storage now based off mercurial module, not 'hg' executible.

This should make repeated calls to Hg storage instances _much_ faster,
since we avoid repeatedly loading and tearing down a python subprocess.
For example, the testsuite runs ~6x faster on my box.

Here's a run with the old Hg implementation:
  $ python test.py libbe.storage.vcs.hg
  ...
  =================================
  ERROR: test_get_previous_children
  ---------------------------------
  Traceback (most recent call last):
    ...
  NotImplementedError
  
  ---------------------------------
  Ran 49 tests in 133.285s
  
  FAILED (errors=1)

A run with the new implementation gives the same results, except for:
  Ran 49 tests in 22.328s

2 files changed, 40 insertions, 22 deletions

diff --git a/libbe/storage/vcs/bzr.py b/libbe/storage/vcs/bzr.py
index 397267a..7335861 100644
--- a/libbe/storage/vcs/bzr.py
+++ b/libbe/storage/vcs/bzr.py
@@ -51,7 +51,7 @@ def new():
 
 class Bzr(base.VCS):
     name = 'bzr'
-    client = None # bzrlib
+    client = None # bzrlib module
 
     def __init__(self, *args, **kwargs):
         base.VCS.__init__(self, *args, **kwargs)
diff --git a/libbe/storage/vcs/hg.py b/libbe/storage/vcs/hg.py
index d2d3281..373dfd2 100644
--- a/libbe/storage/vcs/hg.py
+++ b/libbe/storage/vcs/hg.py
@@ -21,10 +21,21 @@
 Mercurial (hg) backend.
 """
 
+try:
+    # enable importing on demand to reduce startup time
+    from mercurial import demandimport; demandimport.enable()
+    import mercurial
+    import mercurial.version
+    import mercurial.dispatch
+    import mercurial.ui
+except ImportError:
+    mercurial = None
 import os
 import os.path
 import re
 import shutil
+import StringIO
+import sys
 import time # work around http://mercurial.selenic.com/bts/issue618
 
 import libbe
@@ -41,7 +52,7 @@ def new():
 
 class Hg(base.VCS):
     name='hg'
-    client='hg'
+    client=None # mercurial module
 
     def __init__(self, *args, **kwargs):
         base.VCS.__init__(self, *args, **kwargs)
@@ -49,13 +60,26 @@ class Hg(base.VCS):
         self.__updated = [] # work around http://mercurial.selenic.com/bts/issue618
 
     def _vcs_version(self):
-        status,output,error = self._u_invoke_client('--version')
-        return output
+        if mercurial == None:
+            return None
+        return mercurial.version.get_version()
+
+    def _u_invoke_client(self, *args, **kwargs):
+        if 'cwd' not in kwargs:
+            kwargs['cwd'] = self.repo
+        assert len(kwargs) == 1, kwargs
+        ui = mercurial.ui.ui(interactive=False)
+        fullargs = ['--cwd', kwargs['cwd']]
+        fullargs.extend(args)
+        stdout = sys.stdout
+        tmp_stdout = StringIO.StringIO()
+        sys.stdout = tmp_stdout
+        mercurial.dispatch._dispatch(ui, fullargs)
+        sys.stdout = stdout
+        return tmp_stdout.getvalue().rstrip('\n')
 
     def _vcs_get_user_id(self):
-        status,output,error = self._u_invoke_client(
-            'showconfig', 'ui.username')
-        return output.rstrip('\n')
+        return self._u_invoke_client('showconfig', 'ui.username')
 
     def _vcs_detect(self, path):
         """Detect whether a directory is revision-controlled using Mercurial"""
@@ -64,8 +88,7 @@ class Hg(base.VCS):
         return False
 
     def _vcs_root(self, path):
-        status,output,error = self._u_invoke_client('root', cwd=path)
-        return output.rstrip('\n')
+        return self._u_invoke_client('root', cwd=path)
 
     def _vcs_init(self, path):
         self._u_invoke_client('init', cwd=path)
@@ -88,13 +111,11 @@ class Hg(base.VCS):
         if revision == None:
             return base.VCS._vcs_get_file_contents(self, path, revision)
         else:
-            status,output,error = \
-                self._u_invoke_client('cat', '-r', revision, path)
-            return output
+            return self._u_invoke_client('cat', '-r', revision, path)
 
     def _vcs_commit(self, commitfile, allow_empty=False):
         args = ['commit', '--logfile', commitfile]
-        status,output,error = self._u_invoke_client(*args)
+        output = self._u_invoke_client(*args)
         # work around http://mercurial.selenic.com/bts/issue618
         strings = ['nothing changed']
         if self._u_any_in_string(strings, output) == True \
@@ -102,7 +123,7 @@ class Hg(base.VCS):
             time.sleep(1)
             for path in self.__updated:
                 os.utime(os.path.join(self.repo, path), None)
-            status,output,error = self._u_invoke_client(*args)
+            output = self._u_invoke_client(*args)
         self.__updated = []
         # end work around
         if allow_empty == False:
@@ -115,14 +136,11 @@ class Hg(base.VCS):
         if index > 0:
             index -= 1
         args = ['identify', '--rev', str(int(index)), '--%s' % style]
-        kwargs = {'expect': (0,255)}
-        status,output,error = self._u_invoke_client(*args, **kwargs)
-        if status == 0:
-            id = output.strip()
-            if id == '000000000000':
-                return None # before initial commit.
-            return id
-        return None
+        output = self._u_invoke_client(*args)
+        id = output.strip()
+        if id == '000000000000':
+            return None # before initial commit.
+        return id
 
 
 if libbe.TESTING == True: