Intermittent failure

On some runs, freuqnetly enough, this completely erases the existing code, and replaces it with license text.

My guess is that there's perhaps a race condition somewhere in the code.

To replicate this, the following works for me -

1. I invoke it like below (on a repo with just about 15 files) -

```
find | xargs ~/go/bin/addlicense
```

I have about 15 files.

There's a good chance that this erases one of my files with just the license. If it didn't -

2. Git reset to remove the lincense again -

```
git reset --hard
```

And go to step 1 until you see this happening.

Start by reproducing the failure with `find | xargs ~/go/bin/addlicense` against a repository containing about 15 files, repeating after `git reset --hard` as described. Trace the addlicense entry point and file-processing behavior during repeated xargs runs, then verify that existing source is never replaced by license text.

Written by the indexing model from the issue text.