feat(third_party/gitignoreSource): Get gitignore-nix from sources (niv)

It's easier to implement readTree/depot polyfills for gitignoreSource
when it's imported from third_party.sources, rather than in a file at
//third_party.gitignoreSource.

Change-Id: I1323f932bd0feeb2c50ccc76397a80e035842992
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12248
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

third_party/sources/sources.json

@@ -11,6 +11,18 @@
         "url": "https://github.com/ryantm/agenix/archive/f6291c5935fdc4e0bef208cfc0dcab7e3f7a1c41.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
+    "gitignore.nix": {
+        "branch": "master",
+        "description": "Nix functions for filtering local git sources",
+        "homepage": "",
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "rev": "637db329424fd7e46cf4185293b9cc8c88c95394",
+        "sha256": "02wxkdpbhlm3yk5mhkhsp3kwakc16xpmsf2baw57nz1dg459qv8w",
+        "type": "tarball",
+        "url": "https://github.com/hercules-ci/gitignore.nix/archive/637db329424fd7e46cf4185293b9cc8c88c95394.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "home-manager": {
         "branch": "master",
         "description": "Manage a user environment using Nix  [maintainer=@rycee] ",