git-gui: Only bind the spellcheck popup suggestion hook once

git-gui: Only bind the spellcheck popup suggestion hook once

If we reconnect to the spellchecker there is no reason to resetup
the binding for button 3 on our text widget to show the suggestion
list (if available).

Plus, by moving it out of _connect and into init we can now break
out of _connect earlier if there is something wrong with the pipe,
for example if the dictionary we were asked to load is not valid.

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