git-gui: remove duplicate entries from .gitconfig's gui.recentrepo

git-gui: remove duplicate entries from .gitconfig's gui.recentrepo

The git gui's recent repo list may become contaminated with duplicate
entries. The git gui would barf when attempting to remove one entry.
Remove them all - there is no option within 'git config' to selectively
remove one of the entries.

This issue was reported on the 'Git User' list
(https://groups.google.com/forum/#!topic/git-users/msev4KsQGFc,
Warning: gui.recentrepo has multiply values while executing).

And also by zosrothko as a Git-for-Windows issue
https://github.com/git-for-windows/git/issues/1014.

On startup the gui checks that entries in the recentrepo list are still
valid repos and deletes thoses that are not. If duplicate entries are
present the 'git config --unset' will barf and this prevents the gui
from starting.

Subsequent patches fix other parts of recentrepo logic used for syncing
internal lists with the external .gitconfig.

Reported-by: Alexey Astakhov <asstv7@gmail.com>
Signed-off-by: Philip Oakley <philipoakley@iee.org>