ssh: Set git protocol version 2 on SSH ControlMaster

According to https://git-scm.com/docs/protocol-v2#_ssh_and_file_transport,
when using SSH, the environment variable GIT_PROTOCOL must be set
when establishing the connection to the git server.

Normally git does this by itself. But in repo-tool where the SSH
connection is managed by the repo-tool, it must be passed in
explicitly instead.

Under some circumstances of environment configuration, this
caused all repo sync commands over ssh to always use
git protocol version 1. Even when git was configured to use
version 2.

Using git protocol v2 can significantly improve fetch speeds,
since it uses server side filtering of refs, reducing the
amount of unneccessary objects to send.

Change-Id: I6d4c3b7300a6090d707480b1a638ed03622fa71a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/411362
Tested-by: Erik Elmeke <erik@haleytek.corp-partner.google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Erik Elmeke <erik@haleytek.corp-partner.google.com>

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