git-gui: Cleanup redundant column management in blame viewer
The code to handle our three different text widgets is a bit on the messy side as we issue the same command on all three widgets one at a time. Adding (or removing) columns from the viewer is messy, as a lot of locations need to have the new column added into the sequence, or removed from it.
We also now delete the tags we create for each commit when we switch to display another "commit:path" pair. This way the text viewer doesn't get bogged down with a massive number of tags as we traverse through history.
Signed-off-by: Shawn O. Pearce <firstname.lastname@example.org>