Try to find diffstat in PATH before calling it

At least on Solaris, /bin/sh will print a "command not found" message
even if we redirect stderr.

mercurial/patch.py

@@ -663,6 +663,8 b" def export(repo, revs, template='hg-%h.p"
         single(repo.lookup(rev), seqno+1, fp)
 
 def diffstat(patchlines):
+    if not util.find_in_path('diffstat', os.environ.get('PATH', '')):
+        return
     fd, name = tempfile.mkstemp(prefix="hg-patchbomb-", suffix=".txt")
     try:
         p = popen2.Popen3('diffstat -p1 -w79 2>/dev/null > ' + name)