Fix diff highlighting of removed lines starting with -- and added lines starting with ++

[koutcher]

When working with Ada source code (where comments start with --), the diff highlighting is messed up whenever a comment starting at the beginning of the line is removed because tig takes it either for the DEL_FILE or the START prefix.

It should close #875 and #871.

Added more changes closing #804, #762 and #866.

[jonas]

Wow, thanks a lot Thomas!

[odnoletkov]

There is status.showUntrackedFiles Git config option (and corresponding command line options) allowing to override default behavior and say don't show untracked files at all. It is useful for some repositories and some use-cases (like partially tracking home directory). Instead of having a separate option in tig can we read if from the Git configuration to avoid having to duplicate this configuration?

[jonas]

Yes, that's a good idea. Let's do that. Do you want to do a follow-up or should I do it?

[jonas]

Actually, I will have to read the documentation for the option. Currently, Tig shows untracked files by default in the main view. Not sure if it is confusing to have a "status.*" Git option apply to Tig's main view