util: use sys.argv[0] if $HG is unset and 'hg' is not in PATH

This is necessary when the executable name is not 'hg'. For example,
if your system-wide mercurial is name 'hgs', sys.argv[0] is more
accurate than 'hg'.

mercurial/util.py

@@ -335,7 +335,8 b' def hgexecutable():'
         elif main_is_frozen():
             set_hgexecutable(sys.executable)
         else:
-            set_hgexecutable(find_exe('hg') or 'hg')
+            exe = find_exe('hg') or os.path.basename(sys.argv[0])
+            set_hgexecutable(exe)
     return _hgexecutable
 
 def set_hgexecutable(path):