At first, we put the tor_git_revision constant in tor_main.c, so
that we wouldn't have to recompile config.o every time the git
revision changed.  But putting it there had unintended side effect
of forcing every program that wanted to link libor.a (including
test, test-slow, the fuzzers, the benchmarks, etc) to declare their
own tor_git_revision instance.

That's not very nice, especially since we want to start supporting
others who want to link against Tor (see 23846).

So, create a new git_revision.c file that only contains this
constant, and remove the duplicated boilerplate from everywhere
else.

Part of implementing ticket 23845.