fetchgit: take postCheckout to allow collecting revision without leaving .git

pkgs/build-support/fetchgit/default.nix

@@ -83,6 +83,8 @@ lib.makeOverridable (
           # run operations between the checkout completing and deleting the .git
           # directory.
           preFetch ? "",
+          # Shell code executed after `git checkout` and before .git directory removal/sanitization.
+          postCheckout ? "",
           # Shell code executed after the file has been fetched
           # successfully. This can do things like check or transform the file.
           postFetch ? "",
@@ -171,6 +173,7 @@ lib.makeOverridable (
             deepClone
             branchName
             preFetch
+            postCheckout
             postFetch
             fetchTags
             rootDir
@@ -227,6 +230,10 @@ lib.makeOverridable (
 
           inherit preferLocalBuild meta allowedRequisites;
 
+          env = {
+            NIX_PREFETCH_GIT_CHECKOUT_HOOK = finalAttrs.postCheckout;
+          };
+
           passthru = {
             gitRepoUrl = url;
           }

pkgs/build-support/fetchgit/tests.nix

@@ -17,6 +17,16 @@
     sha256 = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";
   };
 
+  collect-rev = testers.invalidateFetcherByDrvHash fetchgit {
+    name = "collect-rev-nix-source";
+    url = "https://github.com/NixOS/nix";
+    rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
+    hash = "sha256-AUTX1K7J5+fojvKYJacXYVV5kio3hrWYz5MCekO6h68=";
+    postCheckout = ''
+      git -C "$out" rev-parse HEAD | tee "$out/revision.txt"
+    '';
+  };
+
   sparseCheckout = testers.invalidateFetcherByDrvHash fetchgit {
     name = "sparse-checkout-nix-source";
     url = "https://github.com/NixOS/nix";