What worked for me is something similar to what rohitmohta is proposing ; in regular DOS command prompt (not on git bash) :
first
git config --global http.proxy http://username:password@proxiURL:proxiPort
and in some cases also
git config --global https.proxy http://username:password@proxiURL:proxiPort
then
git config --global http.sslVerify false
(I confirm it's necessary : if set to true getting "SSL certificate problem: unable to get local issuer certificate" error)
in my case, no need of defining all_proxy variable
and finally
git clone https://github.com/someUser/someRepo.git