git-gui: Use lighter colors in blame view

git-gui: Use lighter colors in blame view

The colors I originally picked out on a Mac OS X system look a
tad too dark on a Windows 2000 system; the greys are dark enough
to make it difficult to read some lines of text and the green used
to highlight the current commit was also difficult to read text on.

I also added a third grey to the mix, to try and help some files
that wind up with a number of neighboring chunks getting the same
colors.

Signed-off-by: Shawn O. Pearce <redacted>