Newly published meta-study in BMJ:
Fruit and vegetable intake and incidence of type 2 diabetes mellitus: systematic review and meta-analysis
Results Six studies met the inclusion criteria; four of these studies also provided separate information on the consumption of green leafy vegetables. Summary estimates showed that greater intake of green leafy vegetables was associated with a 14% (hazard ratio 0.86, 95% confidence interval 0.77 to 0.97) reduction in risk of type 2 diabetes (P=0.01). The summary estimates showed no significant benefits of increasing the consumption of vegetables, fruit, or fruit and vegetables combined.
I have to add a comment:
There is only one supposedly significant, positive correlation found in this study - the one with green veg (hazard ratio 0.86). It is possible [but see also (*)] that the significancy results from pooling four separate studies together. When you look at the individual studies on FIG5 : three are only marginally positive, that is their error estimates touch 1.0, while one large study  (Women’s Health Study) shows the hazard ratio of 1.0 which indicates no effect.
This paper  (Nurses' Health Study data) is also interesting because it is one of the largest and longest of its kind. Again, no benefit overall from vegetable consumption and miniscule benefit from greens alone. It also shows this interesting graph:
Note (*): There is a discrepancy in the data. The resulting hazard ratio may also turn out to be not statistically significant for green vegetables, since the overall P=0.18 as per FIG.5 while at the same time it is written as P=0.01 in the abstract for the same result. Given the wide spread 0.77-0.97, my guess is that the abstract figure of 0.01 may be a typo, but I am not 100% sure.