tests-subrepo-git: emit a different "pwned" message based on the test

Having a single "pwned" message which may or may not be emitted during the
tests for CVE-2016-3068 leads to extra confusion.  Allow each test to emit
a more detailed message based on what the expectations are.

In both cases, we expect a version of git which has had the vulnerability
plugged, as well as a version of mercurial which also knows about
GIT_ALLOW_PROTOCOL.  For the first test, we make sure GIT_ALLOW_PROTOCOL is
unset, meaning that the ext-protocol subrepo should be ignored; if it
isn't, there's either a problem with mercurial or the installed copy of
git.

For the second test, we explicitly allow ext-protocol subrepos, which means
that the subrepo will be accessed and a message emitted confirming that
this was, in fact, our intention.

tests/test-subrepo-git.t

@@ -1135,7 +1135,7 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% >pwned.txt" > .hgsub
+  $ echo "s = [git]ext::sh -c echo% \$PWNED_MSG% >pwned.txt" > .hgsub
   $ git init s
   Initialized empty Git repository in $TESTTMP/tc/malicious-subrepository/s/.git/
   $ cd s
@@ -1146,26 +1146,29 test for Git CVE-2016-3068
   $ hg commit -m "add subrepo"
   $ cd ..
   $ rm -f pwned.txt
-  $ env -u GIT_ALLOW_PROTOCOL hg clone malicious-subrepository malicious-subrepository-protected
+  $ env -u GIT_ALLOW_PROTOCOL \
+  > PWNED_MSG="your git is too old or mercurial has regressed" 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% >pwned.txt
+  cloning subrepo s from ext::sh -c echo% $PWNED_MSG% >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
+  $ env GIT_ALLOW_PROTOCOL=ext PWNED_MSG="you asked for it" hg clone \
+  > malicious-subrepository malicious-subrepository-clone-allowed
   Cloning into '$TESTTMP/tc/malicious-subrepository-clone-allowed/s'... (glob)
   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% >pwned.txt
+  cloning subrepo s from ext::sh -c echo% $PWNED_MSG% >pwned.txt
   abort: git clone error 128 in s (in subrepo s)
   [255]
   $ cat pwned.txt
-  pwned
+  you asked for it