feat(nix/sparseTree): allow specifying subtrees as relative paths

Passed strings will be treated as a relative path below the given root,
which is quite convenient when using depot.path by eliminating a lot of
repetition.

Change-Id: I3da6058094484f4a6ffbb84f89ad4472b502a00c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3704
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

web/bubblegum/default.nix

@@ -12,20 +12,20 @@ let
 
   minimalDepot = sparseTree depot.path [
     # general depot things
-    (depot.path + "/default.nix")
-    (depot.path + "/nix/readTree")
+    "default.nix"
+    "nix/readTree"
     # nixpkgs for lib and packages
-    (depot.path + "/third_party/nixpkgs")
-    (depot.path + "/third_party/overlays")
+    "third_party/nixpkgs"
+    "third_party/overlays"
     # bubblegum and its dependencies
-    (depot.path + "/web/bubblegum")
-    (depot.path + "/nix/runExecline")
-    (depot.path + "/nix/utils")
-    (depot.path + "/nix/sparseTree")
+    "web/bubblegum"
+    "nix/runExecline"
+    "nix/utils"
+    "nix/sparseTree"
     # tvix docs for svg demo
-    (depot.path + "/tvix/docs")
+    "tvix/docs"
     # for blog.nix
-    (depot.path + "/users/sterni/nix")
+    "users/sterni/nix"
   ];
 
   statusCodes = {