tests-subrepo-git: make the "pwned" message output in a stable order

The "pwned" message from this test gets gets sent to stderr, and so may get
emitted in different places from run to run in the rest of mercurial's
output.  This patch forces the message to go to a specific file instead,
whose existence and contents we can examine at a stable point in the test's
execution.

tests/test-subrepo-git.t

@@ -1135,7 +1135,7 b' make sure we show changed files, rather '
 test for Git CVE-2016-3068
   $ hg init malicious-subrepository
   $ cd malicious-subrepository
-  $ echo "s = [git]ext::sh -c echo% pwned% >&2" > .hgsub
+  $ echo "s = [git]ext::sh -c echo% pwned% >pwned.txt" > .hgsub
   $ git init s
   Initialized empty Git repository in $TESTTMP/tc/malicious-subrepository/s/.git/
   $ cd s
@@ -1145,23 +1145,27 b' test for Git CVE-2016-3068'
   $ hg add .hgsub
   $ hg commit -m "add subrepo"
   $ cd ..
+  $ rm -f pwned.txt
   $ env -u GIT_ALLOW_PROTOCOL hg clone malicious-subrepository malicious-subrepository-protected
   Cloning into '$TESTTMP/tc/malicious-subrepository-protected/s'... (glob)
   fatal: transport 'ext' not allowed
   updating to branch default
-  cloning subrepo s from ext::sh -c echo% pwned% >&2
+  cloning subrepo s from ext::sh -c echo% pwned% >pwned.txt
   abort: git clone error 128 in s (in subrepo s)
   [255]
+  $ test -f pwned.txt && cat pwned.txt || true
 
 whitelisting of ext should be respected (that's the git submodule behaviour)
+  $ rm -f pwned.txt
   $ env GIT_ALLOW_PROTOCOL=ext hg clone malicious-subrepository malicious-subrepository-clone-allowed
   Cloning into '$TESTTMP/tc/malicious-subrepository-clone-allowed/s'... (glob)
-  pwned
   fatal: Could not read from remote repository.
   
   Please make sure you have the correct access rights
   and the repository exists.
   updating to branch default
-  cloning subrepo s from ext::sh -c echo% pwned% >&2
+  cloning subrepo s from ext::sh -c echo% pwned% >pwned.txt
   abort: git clone error 128 in s (in subrepo s)
   [255]
+  $ cat pwned.txt
+  pwned