fetchFromGitHub: use fetchgit when the rootDir option is set

This argument was added to fetchgit in #427165,
and it's not supported by fetchzip.

Co-authored-by: Philip Taron <philip.taron@gmail.com>

doc/build-helpers/fetchers.chapter.md

@@ -860,7 +860,14 @@ A number of fetcher functions wrap part of `fetchurl` and `fetchzip`. They are m
 
 To use a different GitHub instance, use `githubBase` (defaults to `"github.com"`).
 
-`fetchFromGitHub` uses `fetchzip` to download the source archive generated by GitHub for the specified revision. If `leaveDotGit`, `deepClone` or `fetchSubmodules` are set to `true`, `fetchFromGitHub` will use `fetchgit` instead. Refer to its section for documentation of these options.
+By default, `fetchFromGitHub` uses `fetchzip` to download GitHub's source archive for the specified revision.
+However, `fetchFromGitHub` will automatically switch to using `fetchgit` in any of these cases:
+
+- `forceFetchGit`, `leaveDotGit`, `deepClone`, `fetchLFS`, or `fetchSubmodules` are set to `true`
+- `sparseCheckout` contains any entries (is a non-empty list)
+- `rootDir` is set to a non-empty string
+
+When `fetchgit` is used, refer to the `fetchgit` section for documentation of its available options.
 
 ## `fetchFromGitLab` {#fetchfromgitlab}
 

pkgs/build-support/fetchgithub/default.nix

@@ -18,7 +18,8 @@ lib.makeOverridable (
     private ? false,
     forceFetchGit ? false,
     fetchLFS ? false,
-    sparseCheckout ? [ ],
+    rootDir ? "",
+    sparseCheckout ? lib.optional (rootDir != "") rootDir,
     githubBase ? "github.com",
     varPrefix ? null,
     meta ? { },
@@ -69,6 +70,7 @@ lib.makeOverridable (
       || deepClone
       || forceFetchGit
       || fetchLFS
+      || (rootDir != "")
       || (sparseCheckout != [ ]);
     # We prefer fetchzip in cases we don't need submodules as the hash
     # is more stable in that case.