git-gui: search 4 directories to improve statistic of gc hint

git-gui: search 4 directories to improve statistic of gc hint

On Windows, git-gui suggests running the garbage collector if it finds
1 or more files in .git/objects/42 (as opposed to 8 files on other
platforms). The probability of that happening if the repo contains
about 100 loose objects is 32%. The probability for the same to happen
when searching 4 directories is only 8%, which is bit more reasonable.

Also remove $objects_limit from the message, because we already know
that we are above (or close to) that limit. Telling the user about
that number does not really give him any useful information.

The following octave script shows the probability for at least m*q
objects to be found in q subdirectories of .git/objects if n is the
total number of objects.

q = 4;
m = [1 2 8];
n = 0:10:2000;

P = zeros(length(n), length(m));
for k = 1:length(n)
        P(k, :) = 1-binocdf(q*m-1, n(k), q/(256-q));
end
plot(n, P);

n \ q   1       4
50      18%     1%
100     32%     8%
200     54%     39%
500     86%     96%

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>