git-gui: Make the line number column slightly wider in blame

git-gui: Make the line number column slightly wider in blame

Most source code files are under 9,999 lines of text, so using a
field width of 5 characters meant that we should have had one char
padding on the left edge (because we right-justify the line number).
Unfortunately when I added the right margin earlier (when I removed
the padding) I ate into the extra character's space, losing the left
margin.  This put the line numbers too close to the commit column in
any file with more than 999 lines in it.

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