Allow to pass -w flag (ignore whitespace changes) to the blame

[Mingun]

git blame allows you to pass -w flag and then if line contains only whitespace changes, git will continue to find previous commit that affect it. Currently SourceGit does not use that flag when you open Blame view. I suggest to add Ignore whitespace changes checkbox to the Blame view which will recalculate blame information by appending / removing -w flag.