diffparse: simplify and fix reverting of worktree diffs

Recalculate new counts for both the start and end ranges, which fixes
how we handle deletions.

Adjust the "is the diff line selected?" metric so that we properly catch
when only the first character of a line is selected.

Signed-off-by: David Aguilar <davvid@gmail.com>
README.md: mention how to build git-cola.app

Signed-off-by: David Aguilar <davvid@gmail.com>