tools: close channel if we fail during walking

If the user runs git lfs track (or almost any command), we'll try to
clean up any temporary files that may be left around.  When doing so, we
walk the temporary directory, looking for any files.

Normally, this works great, but when the user is in a repository that is
owned by another user, the temporary directory may not have been created
and in such a case, our walk may fail.  If so, that's fine, but when we
return an error, we need to be sure to clean up the WaitGroup and close
the channel.  If we don't, the tools.fastWalkCallback routine will hang
waiting for the channel and we won't make any more progress, so the
program will just hang.  Let's defer the Wait call so we always clean up
on the way out.

tools/filetools.go

@@ -304,14 +304,15 @@ func fastWalkWithExcludeFiles(rootDir string) *fastWalker {
 	}
 
 	go func() {
+		defer w.Wait()
+
 		dirFi, err := os.Stat(w.rootDir)
 		if err != nil {
 			w.ch <- fastWalkInfo{Err: err}
 			return
 		}
 
 		w.Walk(true, "", dirFi, excludePaths)
-		w.Wait()
 	}()
 	return w
 }