git-gui: Rename fields in blame viewer to better descriptions

git-gui: Rename fields in blame viewer to better descriptions

Calling the commit message pane $w_cmit is a tad confusing when
we also have the $w_cgrp column that shows the abbreviated SHA-1s.

So w_cmit -> w_cviewer, as it is the "commit viewer"; and
w_cgrp -> w_amov as it is the "annotated commit + move tracking"
column.  Also changed line_data -> amov_data, as that list is
exactly the results shown in w_amov.

Why call the column "move tracking"?  Because this column holds
data from "git blame -M -C".  I'm considering adding an additional
column that holds the data from "git blame" without -M/-C, showing
who did the copy/move, and when they did it.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>