git-gui: minor spelling fix and string factorisation.

git-gui: minor spelling fix and string factorisation.

Properly spell "successful" and slightly rewrite a couple of strings
that actually say the same thing in order to reduce translation work.

Update .pot and .po files accordingly since no new translation is
required.

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