git gui: cope with duplicates in _get_recentrepo

_get_recentrepo will fail if duplicate invalid entries are present
in the recentrepo config list. The previous commit fixed the
'git config' limitations in _unset_recentrepo by unsetting all config
entries, however this code would fail on the second attempt to unset it.

Refactor the code to pre-sort and de-duplicate the recentrepo list to
avoid a potential second unset attempt.

Signed-off-by: Philip Oakley <philipoakley@iee.org>

lib/choose_repository.tcl

@@ -235,14 +235,14 @@ method _invoke_next {} {
 
 proc _get_recentrepos {} {
 	set recent [list]
-	foreach p [get_config gui.recentrepo] {
+	foreach p [lsort -unique [get_config gui.recentrepo]] {
 		if {[_is_git [file join $p .git]]} {
 			lappend recent $p
 		} else {
 			_unset_recentrepo $p
 		}
 	}
-	return [lsort $recent]
+	return $recent
 }
 
 proc _unset_recentrepo {p} {