tests: add req on bash for test-transaction-rollback-on-sigpipe (issue6429)

I think we could work around this by rewriting the helper scripts in
Python, but I don't want to deal with that now and this should prevent
failures due to a lack of bash.

Differential Revision: https://phab.mercurial-scm.org/D10732

tests/hghave.py

@@ -1129,3 +1129,8 b' def has_lzma():'
         return True
     except ImportError:
         return False
+
+
+@check("bash", "bash shell")
+def has_bash():
+    return matchoutput("bash -c 'echo hi'", b'^hi$')

tests/test-transaction-rollback-on-sigpipe.t

@@ -1,3 +1,4 b''
+#require bash
 Test that, when an hg push is interrupted and the remote side recieves SIGPIPE,
 the remote hg is able to successfully roll back the transaction.