git_command: set GIT_HTTP_USER_AGENT on all requests

We've been setting the User-Agent header when making connections
from repo itself, but not when running git (as the latter will set
up User-Agent itself).  Our Gerrit/Git admins say it'll be helpful
if we pass through the repo version settings even when running git.

We currently set GIT_HTTP_USER_AGENT and not GIT_USER_AGENT as it's
unclear if the extended form works over all protocols.  We can wait
for a user request.

Bug: https://crbug.com/gerrit/11144
Change-Id: I21d293f49534058dbc23225152451df26c5b7bef
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239233
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>