Add tests for supporting git submodules and fix issues

The `RepoRecipe` type now takes a type level argument which tells the arbitrary
instance whether to allow submodules in the recipe or not.

cabal-install/src/Distribution/Client/VCS.hs

@@ -63,7 +63,7 @@ import qualified Data.Char as Char
 import qualified Data.List as List
 import qualified Data.Map  as Map
 import System.FilePath
-         ( takeDirectory )
+         ( takeDirectory, (</>) )
 import System.Directory
          ( doesDirectoryExist
          , removeDirectoryRecursive
@@ -386,11 +386,12 @@ vcsGit =
     vcsCloneRepo verbosity prog repo srcuri destdir =
         [ programInvocation prog cloneArgs ]
         -- And if there's a tag, we have to do that in a second step:
-     ++ [ (programInvocation prog (resetArgs tag)) {
-            progInvokeCwd = Just destdir
-          }
-        | tag <- maybeToList (srpTag repo) ]
+     ++ [ git (resetArgs tag) | tag <- maybeToList (srpTag repo) ]
+     ++ [ git (["submodule", "sync", "--recursive"] ++ verboseArg)
+        , git (["submodule", "update", "--init", "--force", "--recursive"] ++ verboseArg)
+        ]
       where
+        git args   = (programInvocation prog args) {progInvokeCwd = Just destdir}
         cloneArgs  = ["clone", srcuri, destdir]
                      ++ branchArgs ++ verboseArg
         branchArgs = case srpBranch repo of
@@ -419,6 +420,15 @@ vcsGit =
         if exists
           then git localDir                 ["fetch"]
           else git (takeDirectory localDir) cloneArgs
+        -- Before trying to checkout other commits, all submodules must be
+        -- de-initialised and the .git/modules directory must be deleted. This
+        -- is needed because sometimes `git submodule sync` does not actually
+        -- update the submodule source URL. Detailed description here:
+        -- https://git.coop/-/snippets/85
+        git localDir ["submodule", "deinit", "--force", "--all"]
+        let gitModulesDir = localDir </> ".git" </> "modules"
+        gitModulesExists <- doesDirectoryExist gitModulesDir
+        when gitModulesExists $ removeDirectoryRecursive gitModulesDir
         git localDir resetArgs
         git localDir ["submodule", "sync", "--recursive"]
         git localDir ["submodule", "update", "--force", "--init", "--recursive"]