Use $GIT_INDEX_FILE instead of --index-output to avoid lockfile problem

The lockfile gets renamed to the final destination after the operation
finishes. But it must be located in the same filesystem, which prevents
us from using /tmp.

models/pull.go

@@ -437,8 +437,8 @@ func (pr *PullRequest) testPatch() (err error) {
 
 	var stderr string
 	_, stderr, err = process.ExecDirEnv(-1, "", fmt.Sprintf("testPatch (git read-tree): %d", pr.BaseRepo.ID),
-		[]string{"GIT_DIR=" + pr.BaseRepo.RepoPath()},
-		"git", "read-tree", "--index-output", indexTmpPath, pr.BaseBranch)
+		[]string{"GIT_DIR=" + pr.BaseRepo.RepoPath(), "GIT_INDEX_FILE=" + indexTmpPath},
+		"git", "read-tree", pr.BaseBranch)
 	if err != nil {
 		return fmt.Errorf("git read-tree --index-output=%s %s: %v - %s", indexTmpPath, pr.BaseBranch, err, stderr)
 	}