bugfix: 'submodules' attribute should imply git fetch

- 'submodules' wasn't properly declared as an optional attribute for git
  fetcher

- add it and add a test.

lib/spack/spack/fetch_strategy.py

@@ -596,7 +596,7 @@ class GitFetchStrategy(VCSFetchStrategy):
     """
     enabled = True
     url_attr = 'git'
-    optional_attrs = ['tag', 'branch', 'commit']
+    optional_attrs = ['tag', 'branch', 'commit', 'submodules']
 
     def __init__(self, **kwargs):
         # Discards the keywords in kwargs that may conflict with the next call

lib/spack/spack/test/packages.py

@@ -341,6 +341,13 @@ def test_git_url_top_level_git_versions(mock_packages, config):
     assert fetcher.commit == 'abc34'
     assert fetcher.branch is None
 
+    fetcher = spack.fetch_strategy.for_package_version(pkg, 'submodules')
+    assert isinstance(fetcher, spack.fetch_strategy.GitFetchStrategy)
+    assert fetcher.url == 'https://example.com/some/git/repo'
+    assert fetcher.tag is None
+    assert fetcher.commit is None
+    assert fetcher.branch is None
+
     fetcher = spack.fetch_strategy.for_package_version(pkg, 'develop')
     assert isinstance(fetcher, spack.fetch_strategy.GitFetchStrategy)
     assert fetcher.url == 'https://example.com/some/git/repo'