git-gui: Automatically expand the line number column as needed

git-gui: Automatically expand the line number column as needed

After we finish reading a chunk of data from the file stream
we know how many digits we need in the line number column to
show the current maximum line number.  If our line number column
isn't wide enough, we should expand it out to the correct width.

Any file over our default allowance of 5 digits (99,999 lines)
is so large that the slight UI "glitch" when we widen the column
out is trivial compared to the time it will take Git to fully do
the annotations.

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