git-gui: Simplify consecutive lines that come from the same commit

git-gui: Simplify consecutive lines that come from the same commit

If two consecutive lines in the final file came from the same commit
then we store a "|" in the first column rather than the commit id,
for the second and subsequent lines in that block.  This cleans up
the interface so runs associated with the same commit can be more
easily seen visually.

We also now use the abbreviation "work" for the uncommitted stuff in
your working directory, rather than "0000".  This looks nicer to the
eyes and explains pretty quickly what is going on.

There was also a minor bug in the commit abbreviation column for the
last line of the file.  This is now also fixed.

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