run-test: clarify the error with a bad --with-hg is passed

This helped me to understand what was going on when I got into trouble.

Differential Revision: https://phab.mercurial-scm.org/D11046

tests/run-tests.py

@@ -728,7 +728,9 def parseargs(args, parser):
         ):
             parser.error('--with-hg must specify an executable hg script')
         if os.path.basename(options.with_hg) not in [b'hg', b'hg.exe']:
-            sys.stderr.write('warning: --with-hg should specify an hg script\n')
+            msg = 'warning: --with-hg should specify an hg script, not: %s\n'
+            msg %= _bytes2sys(os.path.basename(options.with_hg))
+            sys.stderr.write(msg)
             sys.stderr.flush()
 
     if (options.chg or options.with_chg) and WINDOWS:

tests/test-run-tests.t

@@ -23,7 +23,7 error paths
 #if symlink
   $ ln -s `which true` hg
   $ "$PYTHON" $TESTDIR/run-tests.py --with-hg=./hg
-  warning: --with-hg should specify an hg script
+  warning: --with-hg should specify an hg script, not: true
   running 0 tests using 0 parallel processes 
   
   # Ran 0 tests, 0 skipped, 0 failed.