Is there a way to prevent git from changing permissions and ownership on pull?

This sounds like the user you're running has the default group set to yuri. You can confirm this like so:

$ id -a
uid=1000(saml) gid=1000(saml) groups=1000(saml),10(wheel),989(wireshark)

The UID of your account is this: uid=1000(saml) whereas the default group is git=1000(saml) and any secondary groups are thereafter.

NOTE: If you want the git clone to have specific ownership, then you have at least 2 options.

Option #1

Set a parent directory with the permissions as you want like so:

$ mkdir topdir
$ chgrp http topdir
$ chmod g+s topdir

$ cd topdir
$ git clone ....

This forced the directory topdir to enforce any child directories underneath it to have the group http applied. This will work by in large but can lead to problems, since if you move files into this git clone workspace, those files will not have their groups enforced by the changes made above.

Option #2

Prior to doing work, change your default group to http like so:

$ newgrp http
$ git clone ...

This method will force any new files created to have their group set to http instead of your normal default group of yuri, but this will only work so long as you remember to do a newgrp prior to working in this workspace.

Other options

If neither of these seem acceptable you can try using ACLs instead on the git workspace directory. These are discussed in multiple Q&A's on this site, such as in this Q&A titled: Getting new files to inherit group permissions on Linux.

The solution I use is to run the command as the user that has the permissions you want to keep:

sudo -u user command

This keeps the permissions from changing. I use it when updating git repositories on my VPS, while keeping the file permissions set to the webserver user.

See also the same question here.