Add safeguard around git diff scan

[mpaulmier]

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

Fixes #145 && #127 (reopening of #154)

[alphapapa]

I've rebased this on master, but now I see a minor problem: if the branch-diff scanner takes quite a long time (e.g. if there is a lot of diff output to scan), and the branch-diff scan is aborted by refreshing the status buffer before the scan finished, the branch-diff section will be inserted with whatever matches are found in the partially complete scan output. Then when the uninterrupted scan finishes, it inserts another section with all of the matches. This is confusing, as the first section implies that there are only that many matches, and the user may not know that there's another scan still running, which may insert a section with a different number of matches.

So we need to find a way to prevent the aborted scan from inserting results.

[mpaulmier]

Hello, sorry for the late response, I haven't looked at this package for quite some time now tbh so I haven't had the chance to test my branch again. I'll see if I can give it a try on a weekend and fix the issue you are reporting.

I've quickly tested on a project at work today and wasn't able to observe the behaviour you're describing. Maybe the diff wasn't large enough. To me the TODOs sections don't appear at all until the scans are done.