Feature "Diff From Branch" on BranchManager

Original patch by Carlos Nihelton, thanks! I performed some code cleanups.

Summary:
I was annoyed by the fact that I couldn't create diffs in KDevelop (using git plugin at least) after having committed the changes to any branch. So I added a button on BranchManager UI to allow creation of diffs between the current branch and any branch selected in the list view. In current implementation it uses the command line "git diff --no-color --no-ext-diff --cached <branch> -- <path>", thus, it includes the staging area. If it's clean, it differs from the last commit.

This diff was generated using this feature.

Reviewers: brauch, kfunk, apol

Reviewed By: brauch, kfunk

Subscribers: brauch, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3073