Improve git submodule error reporting

src/libfetchers/fetchers.cc

@@ -220,6 +220,16 @@ std::pair<StorePath, Input> Input::fetch(ref<Store> store) const
     return {std::move(storePath), input};
 }
 
+std::pair<ref<InputAccessor>, Input> Input::getAccessor(ref<Store> store) const
+{
+    try {
+        return scheme->getAccessor(store, *this);
+    } catch (Error & e) {
+        e.addTrace({}, "while fetching the input '%s'", to_string());
+        throw;
+    }
+}
+
 Input Input::applyOverrides(
     std::optional<std::string> ref,
     std::optional<Hash> rev) const

src/libfetchers/fetchers.hh

@@ -83,6 +83,8 @@ public:
      */
     std::pair<StorePath, Input> fetch(ref<Store> store) const;
 
+    std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store) const;
+
     Input applyOverrides(
         std::optional<std::string> ref,
         std::optional<Hash> rev) const;

src/libfetchers/git.cc

@@ -605,7 +605,7 @@ struct GitInputScheme : InputScheme
                 attrs.insert_or_assign("rev", submoduleRev.gitRev());
                 auto submoduleInput = fetchers::Input::fromAttrs(std::move(attrs));
                 auto [submoduleAccessor, submoduleInput2] =
-                    submoduleInput.scheme->getAccessor(store, submoduleInput);
+                    submoduleInput.getAccessor(store);
                 mounts.insert_or_assign(submodule.path, submoduleAccessor);
             }
 
@@ -649,7 +649,7 @@ struct GitInputScheme : InputScheme
                 attrs.insert_or_assign("url", submodulePath.abs());
                 auto submoduleInput = fetchers::Input::fromAttrs(std::move(attrs));
                 auto [submoduleAccessor, submoduleInput2] =
-                    submoduleInput.scheme->getAccessor(store, submoduleInput);
+                    submoduleInput.getAccessor(store);
 
                 /* If the submodule is dirty, mark this repo dirty as
                    well. */