tests: add run-tests --changed option for running tests changed in revisions

Convenient when polishing patches and changing details of how they change test
output.

This will probably break in weird ways for revsets with special quoting ... but
it is good enough for run-tests.

Usage example:
  yes | ./run-tests.py -li --changed qparent

tests/run-tests.py

@@ -141,6 +141,8 b' def parseargs():'
         help="skip tests listed in the specified blacklist file")
     parser.add_option("--whitelist", action="append",
         help="always run tests listed in the specified whitelist file")
+    parser.add_option("--changed", type="string",
+        help="run tests that are changed in parent rev or working directory")
     parser.add_option("-C", "--annotate", action="store_true",
         help="output files annotated with coverage")
     parser.add_option("-c", "--cover", action="store_true",
@@ -1181,7 +1183,13 b' def main():'
     checktools()
 
     if not args:
-        args = os.listdir(".")
+        if options.changed:
+            proc = Popen4('hg st --rev "%s" -man0 .' % options.changed,
+                          None, 0)
+            stdout, stderr = proc.communicate()
+            args = stdout.strip('\0').split('\0')
+        else:
+            args = os.listdir(".")
 
     tests = [t for t in args
              if t.startswith("test-")