chg: make `chg --cwd $anything -R $relativepath` behave better

We're seeing this kind of issue:

```
  $ (cd repo1; chg --cwd ../repo2 -R .)
  chg: abort: too many redirections.
  Please make sure $path/hg is not a wrapper which changes sensitive environment variables before executing hg. If you have to use a wrapper, wrap chg instead of hg.
  [255]
```

chgserver is interpreting -R as relative to its cwd, instead of
relative to --cwd like regular hg does, which I think causes an
inconsistency leading to the above.

Differential Revision: https://phab.mercurial-scm.org/D12579

mercurial/dispatch.py

@@ -991,6 +991,8 b' def _getlocal(ui, rpath, wd=None):'
             lui.readconfig(os.path.join(path, b".hg", b"hgrc-not-shared"), path)
 
     if rpath:
+        if wd is not None:
+            rpath = os.path.join(wd, rpath)
         path = urlutil.get_clone_path(lui, rpath)[0]
         lui = ui.copy()
         if rcutil.use_repo_hgrc():

tests/test-chg.t

@@ -433,6 +433,15 b' check server log:'
   YYYY/MM/DD HH:MM:SS (PID)> log -R cached
   YYYY/MM/DD HH:MM:SS (PID)> loaded repo into cache: $TESTTMP/cached (in  ...s)
 
+-R is interpreted relative to --cwd
+
+  $ hg init repo1
+  $ hg init repo2
+  $ printf "[alias]\nx=y\n" >> repo1/.hg/hgrc
+  $ printf "[alias]\nx=z\n" >> repo2/.hg/hgrc
+  $ (cd repo1; chg --cwd ../repo2 -R . show alias.x)
+  z
+
 Test that chg works (sets to the user's actual LC_CTYPE) even when python
 "coerces" the locale (py3.7+)
 
@@ -537,3 +546,4 b" FIXME: Run 4 should not be >3x Run 1's n"
   $ filteredchg log -r . --no-profile
   $ filteredchg log -r .
   Sample count: * (glob)
+  $ cd ../