[BUG] git gui alert dialogs interfere with each other

(Seen in 1.7.12.3)

When launching git gui in a repo where there are 1. lots of (>5000) stashed changes
(to do a tree move), and 2. many non-packed files, git gui throws 2 alert dialogs:
one saying that it will only show 5000 changed files and ignore the others, which
very quickly hidden by the offer to repack.  If I decline repacking, then both
dialogs are discarded, and the first one, which is quite critical, may not have been
seen at all by the user.

-- 
Yann Dirson - Bertin Technologies