GitRepo::fetch(): Ignore $GIT_DIR

Fixes #12325.

Author: edolstra

src/libfetchers/git-utils.cc

@@ -545,7 +545,7 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
         //       then use code that was removed in this commit (see blame)
 
         auto dir = this->path;
-        Strings gitArgs{"-C", dir.string(), "fetch", "--quiet", "--force"};
+        Strings gitArgs{"-C", dir.string(), "--git-dir", ".", "fetch", "--quiet", "--force"};
         if (shallow)
             append(gitArgs, {"--depth", "1"});
         append(gitArgs, {std::string("--"), url, refspec});

tests/functional/common/vars.sh

@@ -60,6 +60,7 @@ unset XDG_DATA_HOME
 unset XDG_CONFIG_HOME
 unset XDG_CONFIG_DIRS
 unset XDG_CACHE_HOME
+unset GIT_DIR
 
 export IMPURE_VAR1=foo
 export IMPURE_VAR2=bar