Got reason of the problem, it was gnutls package. It's working weird behind a proxy. But openssl is working fine even in weak network. So workaround is that we should compile git with openssl. To do this, run the following commands:
sudo apt-get install build-essential fakeroot dpkg-dev mkdir ~/git-openssl cd ~/git-openssl sudo apt-get source git sudo apt-get build-dep git sudo apt-get install libcurl4-openssl-dev sudo dpkg-source -x git_1.7.9.5-1.dsc cd git_1.7.9.5 8.
Then, edit debian/control file (run the command: gksu gedit debian/control) and replace all instances of libcurl4-gnutls-dev with libcurl4-openssl-dev
sudo dpkg-buildpackage -rfakeroot -b
(if it's failing on test, you can remove the line TEST=test from the file debian/rules)
sudo dpkg -i ../git_1.7.9.5-1_i386.deb