Consistent patch ordering (#10879)

* preserve the order in which patches are applied by packages (in spite of grouping them by 'when')

* add tests confirming patch order

var/spack/repos/builtin.mock/packages/dep-diamond-patch-mid1/mid1.patch

@@ -0,0 +1 @@
+mid1

var/spack/repos/builtin.mock/packages/dep-diamond-patch-mid1/package.py

@@ -0,0 +1,30 @@
+# Copyright 2013-2019 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class DepDiamondPatchMid1(Package):
+    r"""Package that requires a patch on a dependency
+
+  W
+ / \
+X   Y
+ \ /
+  Z
+
+    This is package X
+    """
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/patch-a-dependency-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    # single patch file in repo
+    depends_on('patch', patches='mid1.patch')
+
+    def install(self, spec, prefix):
+        pass

var/spack/repos/builtin.mock/packages/dep-diamond-patch-mid2/package.py

@@ -0,0 +1,33 @@
+# Copyright 2013-2019 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class DepDiamondPatchMid2(Package):
+    r"""Package that requires a patch on a dependency
+
+  W
+ / \
+X   Y
+ \ /
+  Z
+
+    This is package Y
+    """
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/patch-a-dependency-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    # single patch file in repo
+    depends_on('patch', patches=[
+        patch('http://example.com/urlpatch.patch',
+              sha256='mid21234abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234'),  # noqa: E501
+    ])
+
+    def install(self, spec, prefix):
+        pass

var/spack/repos/builtin.mock/packages/dep-diamond-patch-top/package.py

@@ -0,0 +1,32 @@
+# Copyright 2013-2019 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class DepDiamondPatchTop(Package):
+    r"""Package that requires a patch on a dependency
+
+  W
+ / \
+X   Y
+ \ /
+  Z
+
+    This is package W
+    """
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/patch-a-dependency-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    # single patch file in repo
+    depends_on('patch', patches='top.patch')
+    depends_on('dep-diamond-patch-mid1')
+    depends_on('dep-diamond-patch-mid2')
+
+    def install(self, spec, prefix):
+        pass

var/spack/repos/builtin.mock/packages/dep-diamond-patch-top/top.patch

@@ -0,0 +1 @@
+top

var/spack/repos/builtin.mock/packages/patch/package.py

@@ -13,9 +13,10 @@ class Patch(Package):
     url = "http://www.example.com/patch-1.0.tar.gz"
 
     version('1.0', '0123456789abcdef0123456789abcdef')
+    version('2.0', '0123456789abcdef0123456789abcdef')
 
     patch('foo.patch')
-    patch('bar.patch')
+    patch('bar.patch', when='@2:')
     patch('baz.patch')
 
     def install(self, spec, prefix):