git-gui: Keep repo_config(gui.recentrepos) and .gitconfig in sync

git-gui: Keep repo_config(gui.recentrepos) and .gitconfig in sync

When the number of recent repo's gets to ten there can be a
situation where an item is removed from the .gitconfig file via
a call to git config --unset, but the internal representation of
that file (repo_config(gui.recentrepo)) is not updated.  Then a
subsequent attempt to remove an item from the list fails because
git-gui attempts to call --unset on a value that has already
been removed.  This leads to duplicates in the .gitconfig file,
which then also cause errors if the git-gui tries to --unset them
(rather than using --unset-all. --unset-all is not used because it
is not expected that duplicates should ever be allowed to exist.)

When loading the list of recent repositories (proc _get_recentrepos)
if a repo in the list is not considered a valid git reposoitory
then we should go ahead and remove it so it doesn't take up a slot
in the list (since we limit to 10 items). This will prevent a bunch
of invalid entries in the list (which are not shown) from making
valid entries dissapear off the list even when there are less than
ten valid entries.

See: http://code.google.com/p/msysgit/issues/detail?id=362
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>