gitSource: add subdir option

README.md

@@ -196,6 +196,11 @@ pkgs.mkNginxServer {
 for development package nix files, computes the source set of files tracked by git at the given root
 path
 
+arguments:
+- `root`: the root of the git repo, where `.git` is located
+- `subdir`, optional: a subdirectory within the git repo. if provided, only files in this
+    subdirectory will go into the final source set
+
 example:
 ```nix
 stdenv.mkDerivation {

lib/git-source/default.nix

@@ -1,8 +1,14 @@
-{ lib }: { root }:
+{ lib }: { root, subdir ? null }:
 let
   fs = lib.fileset;
   sourceFiles = fs.difference
     (fs.gitTracked root)
     (fs.fileFilter (file: file.hasExt "nix") root);
+  finalSourceFiles =
+    if subdir == null then
+      sourceFiles
+    else
+      fs.intersection sourceFiles subdir;
+  finalRoot = if subdir == null then root else subdir;
 in
-  fs.toSource { inherit root; fileset = sourceFiles; }
+  fs.toSource { root = finalRoot; fileset = sourceFiles; }