Fix text diff for 1st commit and deleted files

[fcollonval]

This fixes the display of text diff in history for two cases:

• First commit (no previous commit)
• Commit at which the file is deleted

[ianhi]

Nice!

One note: I think this is still missing one case with deleted files. It works as I'd expected from the history panel, but if you try to get the diff of the file from the commit panel you will get an error

[fcollonval]

Good catch @ianhi. I will extend this PR to cover that case.

Thank for reviewing

[fcollonval]

I added the versioned file being deleted case

[ianhi]

This is great thanks!

[fcollonval]

@meeseeksdev backport to 0.11.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout 0.11.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 09bf40630d9bfb26662d5e83619b2bfce5f57131

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #766: Fix text diff for 1st commit and deleted files'

4. Push to a named branch :

git push YOURFORK 0.11.x:auto-backport-of-pr-766-on-0.11.x

5. Create a PR against branch 0.11.x, I would have named this PR:

"Backport PR #766 on branch 0.11.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.