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>

main.py

@@ -317,7 +317,7 @@ def _UserAgent():
     _user_agent = 'git-repo/%s (%s) git/%s Python/%d.%d.%d' % (
       repo_version,
       os_name,
-      '.'.join(map(str, git.version_tuple())),
+      git.version_tuple().full,
       py_version[0], py_version[1], py_version[2])
   return _user_agent