main: user-agent: include full git version info

We've been truncating the git version info in the user agent to the
first three components.  So given an example `git --version` like
"2.22.0.510.g264f2c817a-goog", we were cutting it down to "2.22.0".
For user-agent purposes, we usually want that full string, so use
the original full value instead.

Bug: https://crbug.com/gerrit/11144
Change-Id: I8ffe3186bdaac96164c34ac835a54bb3fc85527e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/231056


Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>