Don't hardcode bash location

bash may not be installed in /bin, so don't make that assumption.

Change-Id: I04ae786c7558f829ddf9649c127cbba1457dcfd3
diff --git a/contrib/git-exproll.sh b/contrib/git-exproll.sh
index 066c57c..9ad7a85 100644
--- a/contrib/git-exproll.sh
+++ b/contrib/git-exproll.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
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