git: stop setting GIT_HTTP_USER_AGENT

git: stop setting GIT_HTTP_USER_AGENT

Instead of setting and exporting GIT_HTTP_USER_AGENT set the
$chroot/etc/girocco/.gitconfig http.userAgent config item.

Since 72bb4861f18c4d7e (user-agent: also set GIT_HTTP_USER_AGENT
when requested, 2015-02-01), Girocco has also been setting the
GIT_HTTP_USER_AGENT environment variable to provide support for
older Git versions >= 1.7.3 but < 1.7.12 that do not understand
the GIT_USER_AGENT environment variable.

Move this setting into the config item http.userAgent that's
stored in $Girocco::Config::chroot/etc/girocco/.gitconfig instead.

It will be just as effective there as it was in the environment
variable but it will no longer interfere with repository-specific
overrides of the http.userAgent setting that may be needed to
fetch from specific hosts.

As a side effect the code gets a little bit simpler too.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>