I have two different versions of git.
In the 1.6.2 version, git push does not have the -u option. It only appears in the 1.7.x version.
From the docs, the -u is related to the variable
branch.<name>.merge
in git config. This variable is described below:
Defines, together with branch.<name>.remote, the upstream branch
for the given branch. It tells git fetch/git pull which branch to merge.
What is an upstream branch ?