tests: factor external procedures out for portability

Fortunately, "&&" is treated as "execute next, if previous doesn't
fail" both on POSIX and Windows. But keeping portability of
"dirstaterace.command" manually is troublesome.

This patch factors external procedures out as a shell script for
portability. "sh SCRIPT" always allows scripting in POSIX style.

This change is also for convenience. Fixed script name can reduce
command line arguments.

"r" prefix is needed for "sh '$TESTTMP/dirstaterace.sh'", because
$TESTTMP contains backslash on Windows.

tests/test-dirstate-race.t

@@ -57,7 +57,8 b' confused with a file with the exec bit s'
   >     extensions.wrapfunction(context.workingctx, '_checklookup', overridechecklookup)
   > def overridechecklookup(orig, self, files):
   >     # make an update that changes the dirstate from underneath
-  >     self._repo.ui.system(self._repo.ui.config('dirstaterace', 'command'), cwd=self._repo.root)
+  >     self._repo.ui.system(r"sh '$TESTTMP/dirstaterace.sh'",
+  >                          cwd=self._repo.root)
   >     return orig(self, files)
   > EOF
 
@@ -73,8 +74,11 b' XXX Note that this returns M for files t'
 definitely a bug, but the fix for that is hard and the next status run is fine
 anyway.
 
-  $ hg status --config extensions.dirstaterace=$TESTTMP/dirstaterace.py \
-  >   --config dirstaterace.command='rm b && rm -r dir1 && rm d && mkdir d && rm e && mkdir e'
+  $ cat > $TESTTMP/dirstaterace.sh <<EOF
+  > rm b && rm -r dir1 && rm d && mkdir d && rm e && mkdir e
+  > EOF
+
+  $ hg status --config extensions.dirstaterace=$TESTTMP/dirstaterace.py
   M d
   M e
   ! b