util._hgexecutable: default to 'hg'

util._hgexecutable: default to 'hg'

This should make hooks work even if util.set_hgexecutable is not called
(e.g. hgweb.cgi).

Fixes issue625.

--- a/mercurial/util.py	Wed Jul 11 17:40:41 2007 -0300
+++ b/mercurial/util.py	Wed Jul 11 18:15:21 2007 -0300
@@ -540,7 +540,7 @@
 
     return (roots, match, (inc or exc or anypats) and True)
 
-_hgexecutable = None
+_hgexecutable = 'hg'
 
 def set_hgexecutable(path):
     """remember location of the 'hg' executable if easily possible
@@ -549,7 +549,8 @@
     fall back to 'hg' in this case.
     """
     global _hgexecutable
-    _hgexecutable = path and os.path.abspath(path) or 'hg'
+    if path:
+        _hgexecutable = os.path.abspath(path)
 
 def system(cmd, environ={}, cwd=None, onerr=None, errprefix=None):
     '''enhanced shell command execution.