Add git submodule fixes from @bjornfor

This fixes fetching repositories with no submodules and also cleans up
.git files in checkouts.

src/libexpr/local.mk

@@ -9,6 +9,7 @@ libexpr_SOURCES := $(wildcard $(d)/*.cc) $(wildcard $(d)/primops/*.cc) $(d)/lexe
 libexpr_LIBS = libutil libstore libnixrust
 
 libexpr_LDFLAGS =
+
 ifneq ($(OS), FreeBSD)
  libexpr_LDFLAGS += -ldl
 endif

src/libexpr/primops/fetchGit.cc

@@ -6,6 +6,7 @@
 #include "hash.hh"
 #include "tarfile.hh"
 
+#include <filesystem>
 #include <sys/time.h>
 
 #include <regex>
@@ -182,9 +183,13 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
 
         runProgram("git", true, { "-C", tmpDir, "checkout", "--quiet", "FETCH_HEAD" });
         runProgram("git", true, { "-C", tmpDir, "remote", "add", "origin", uri });
-        runProgram("git", true, { "-C", tmpDir, "submodule", "--quiet", "update", "--init" });
+        runProgram("git", true, { "-C", tmpDir, "submodule", "--quiet", "update", "--init", "--recursive" });
 
-        deletePath(tmpDir + "/.git");
+        for (const auto& p : std::filesystem::recursive_directory_iterator(tmpDir)) {
+            if (p.path().filename() == ".git") {
+                std::filesystem::remove_all(p.path());
+            }
+        }
 
         gitInfo.submodules = true;
     } else {

tests/fetchGitSubmodules.sh

@@ -47,4 +47,5 @@ pathWithSubmodules=$(nix eval --raw "(builtins.fetchGit { url = file://$rootRepo
 [[ ! -e $pathWithoutSubmodules/sub/content ]]
 [[ -e $pathWithSubmodules/sub/content ]]
 
-
+# No .git directory or submodule reference files must be left
+test "$(find "$pathWithSubmodules" -name .git)" = ""