fetchFromGitHub: add fetchSubmodules option

This commit extends fetchFromGitHub with ability to fetch GitHub
repositories with submodules, so we can use the function consistently
with all GitHub repositories.

Note it doesn't change the previous behavior.

pkgs/build-support/fetchgit/default.nix

@@ -26,7 +26,7 @@ in
    Cloning branches will make the hash check fail when there is an update.
    But not all patches we want can be accessed by tags.
 
-   The workaround is getting the last n commits so that it's likly that they
+   The workaround is getting the last n commits so that it's likely that they
    still contain the hash we want.
 
    for now : increase depth iteratively (TODO)

pkgs/top-level/all-packages.nix

@@ -203,12 +203,25 @@ in
 
   fetchFromGitHub = {
     owner, repo, rev, name ? "${repo}-${rev}-src",
+    fetchSubmodules ? false,
     ... # For hash agility
-  }@args: fetchzip ({
-    inherit name;
-    url = "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz";
-    meta.homepage = "https://github.com/${owner}/${repo}/";
-  } // removeAttrs args [ "owner" "repo" "rev" ]) // { inherit rev; };
+  }@args:
+  let
+    baseUrl = "https://github.com/${owner}/${repo}";
+    passthruAttrs = removeAttrs args [ "owner" "repo" "rev" "fetchSubmodules" ];
+  in if fetchSubmodules then
+    fetchgit ({
+      inherit name rev fetchSubmodules;
+      url = "${baseUrl}.git";
+    } // passthruAttrs)
+  else
+    # We prefer fetchzip in cases we don't need submodules as the hash
+    # is more stable in that case.
+    fetchzip ({
+      inherit name;
+      url = "${baseUrl}/archive/${rev}.tar.gz";
+      meta.homepage = "${baseUrl}/";
+    } // passthruAttrs) // { inherit rev; };
 
   fetchFromBitbucket = {
     owner, repo, rev, name ? "${repo}-${rev}-src",