Defining TOR_BUILD_TAG and tor_git_revision violates the version spec

When we removed the git revision in #2988 (moved), we allowed vendors to specify TOR_BUILD_TAG instead. But if we specify both TOR_BUILD_TAG and tor_git_revision, get_version() returns a string like this:

Tor 0.2.9.9 (TOR_BUILD_TAG) (git-tor_git_revision)

This violates the version spec, which only allows one set of brackets for EXTRA_INFO. https://gitweb.torproject.org/torspec.git/tree/version-spec.txt#n22

So instead, we should use:

Tor 0.2.9.9 (TOR_BUILD_TAG,git-tor_git_revision)

(We can't use spaces in the EXTRA_INFO.)

We should also write a unit test that checks that our own version passes the directory authority checks.

This isn't serious, because the only programmatic interface that uses this is GETINFO version.