Config: allow custom Git user agent string settings

Config: allow custom Git user agent string settings

The Git user agent string can be used to pinpoint possible
vulnerabilities for an attacker.

While changing/suppressing the Git user agent string does
not eliminate any vulnerabilities that might be present, it
does require the potential attacker to do more work.

Support setting the GIT_USER_AGENT value independently for both
client and server operations so that security conscious sites
may control the advertised value.