dispatch: make "_checkshellalias" reusable regardless of adding aliases

To reduce changes in the subsequent patch fixing issue4355, this patch
makes "_checkshellalias" reusable regardless of adding aliases.

In this patch, alias definitions are added and restored, only when
"precheck=True".

mercurial/dispatch.py

@@ -627,7 +627,12 def _getlocal(ui, rpath):
 
     return path, lui
 
-def _checkshellalias(lui, ui, args):
+def _checkshellalias(lui, ui, args, precheck=True):
+    """Return the function to run the shell alias, if it is required
+
+    'precheck' is whether this function is invoked before adding
+    aliases or not.
+    """
     options = {}
 
     try:
@@ -638,14 +643,18 def _checkshellalias(lui, ui, args):
     if not args:
         return
 
+    if precheck:
     norepo = commands.norepo
     optionalrepo = commands.optionalrepo
     def restorecommands():
         commands.norepo = norepo
         commands.optionalrepo = optionalrepo
-
     cmdtable = commands.table.copy()
     addaliases(lui, cmdtable)
+    else:
+        def restorecommands():
+            pass
+        cmdtable = commands.table
 
     cmd = args[0]
     try: