Add safeguard around git diff scan

[ghost]

This is to prevent the scan to be run multiple times and show more than once in Magit's buffer

Fixes #145 and #127

I just went back to work from vacation and stumbled on this problem again (guess I hit g too often...).

[alphapapa]

Thanks.

[alphapapa]

@mpaulmier What happened here?

[mpaulmier]

Oh yeah, sorry about that. In an attempt to merge my github accounts, I deleted the account that created this PR because it was becoming too annoying to manage both.

I can reopen the PRs (this one and #144) under this account if you want or you can take the code and commit it directly if you want.

EDIT: Funny that the PR was closed, I had one in another project that was kept alive even though the creator is now @ghost

[alphapapa]

I understand. Well, if it's not too much trouble, I'd be grateful if you'd reopen them. I do intend to merge them, I just have a lot of projects right now. I appreciate your patience. (FWIW I've recently merged some 3-year-old PRs on other projects. I do get to them eventually...)