Notice that I'm using git username, so, if you are using gitosis or any other username, just fill in your's!
In console with root user execute this command:
visudo
The "vi" editor will be opened. Add these lines:
Defaults:git !authenticate
git ALL=(www-data) ALL
In result the file (that is opened in "vi" editor by calling "visudo") should look like this:
# /etc/sudoers
#
# This file MUST be edited with the 'visudo' command as root.
#
# See the man page for details on how to write a sudoers file.
#
Defaults env_reset
Defaults:git !authenticate
# Host alias specification
# User alias specification
# Cmnd alias specification
# User privilege specification
root ALL=(ALL) ALL
git ALL=(www-data) ALL
# Allow members of group sudo to execute any command
# (Note that later entries override this, so you might need to move
# it further down)
%sudo ALL=(ALL) ALL
#
#includedir /etc/sudoers.d
Then press CTRL+O to save the file, then press Enter to accept the filename (bla bla bla), then press CTRL+X to close the "vi" editor.
Voila! Now git user can execute commands as www-data user:
sudo -u www-data git pull origin master