Don't lose current line # context e.g. in Source vs. Blame

(a) load a repo in gitx-dev
(b) View > Tree View
(c) pick a long file to see the Source of
(d) scroll down some, more than one page of text
(e) click on the Blame button to switch to that view
(f) Expected: blame will show me the blame around the line i'm already looking at / Actual: it jumps back to line 1, losing my context.
