git-gui: Cleanup redundant column management in blame viewer
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 <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git