tests: protect tests involving git ext::sh with git-ext-sh

tests/test-convert-git.t

@@ -948,7 +948,7 b' damage git repository by renaming a tree'
   $ hg convert git-repo4 git-repo4-broken-hg 2>&1 | grep 'abort:'
   abort: cannot read changes in 1c0ce3c5886f83a1d78a7b517cdff5cf9ca17bdd
 
-#if no-windows
+#if no-windows git19
 
 test for escaping the repo name (CVE-2016-3069)
 

tests/test-subrepo-git.t

@@ -1137,6 +1137,8 b' make sure we show changed files, rather '
   ? s/foobar.orig
   ? s/snake.python.orig
 
+#if git19
+
 test for Git CVE-2016-3068
   $ hg init malicious-subrepository
   $ cd malicious-subrepository
@@ -1178,3 +1180,5 b' whitelisting of ext should be respected '
   [255]
   $ f -Dq pwned.txt
   pwned: you asked for it
+
+#endif