dispatch: defer environment variable resolution in alias commands (BC)

Before this patch, if there are environment variables in an alias command,
they will be expanded immediately when we first see the alias.

This will cause issues with chg, because environment variable updates will
not propagate to expanded arguments.

This patch makes "args" of "cmdalias" a property that will be calculated
every time when accessed.

mercurial/dispatch.py

@@ -384,7 +384,7 b' class cmdalias(object):'
         self.cmdname = ''
         self.definition = definition
         self.fn = None
-        self.args = []
+        self.givenargs = []
         self.opts = []
         self.help = ''
         self.badalias = None
@@ -432,7 +432,7 b' class cmdalias(object):'
                              % (self.name, inst))
             return
         self.cmdname = cmd = args.pop(0)
-        args = map(util.expandpath, args)
+        self.givenargs = args
 
         for invalidarg in ("--cwd", "-R", "--repository", "--repo", "--config"):
             if _earlygetopt([invalidarg], args):
@@ -448,7 +448,6 b' class cmdalias(object):'
             else:
                 self.fn, self.opts = tableentry
 
-            self.args = aliasargs(self.fn, args)
             if self.help.startswith("hg " + cmd):
                 # drop prefix in old-style help lines so hg shows the alias
                 self.help = self.help[4 + len(cmd):]
@@ -462,6 +461,11 b' class cmdalias(object):'
             self.badalias = (_("alias '%s' resolves to ambiguous command '%s'")
                              % (self.name, cmd))
 
+    @property
+    def args(self):
+        args = map(util.expandpath, self.givenargs)
+        return aliasargs(self.fn, args)
+
     def __getattr__(self, name):
         adefaults = {'norepo': True, 'optionalrepo': False, 'inferrepo': False}
         if name not in adefaults:

tests/test-alias.t

@@ -525,6 +525,24 b' invalid global arguments for normal comm'
   (use "hg help" for the full list of commands or "hg -v" for details)
   [255]
 
+environment variable changes in alias commands
+
+  $ cat > $TESTTMP/setcount.py <<EOF
+  > import os
+  > def uisetup(ui):
+  >     os.environ['COUNT'] = '2'
+  > EOF
+
+  $ cat >> $HGRCPATH <<'EOF'
+  > [extensions]
+  > setcount = $TESTTMP/setcount.py
+  > [alias]
+  > showcount = log -T "$COUNT\n" -r .
+  > EOF
+
+  $ COUNT=1 hg showcount
+  2
+
 This should show id:
 
   $ hg --config alias.log='id' log