Gitian builds: first repo clone should checkout right tag #4269

If one follows current (and past) instructions for building bitcoin with gitian-builder (doc/gitian-building.md), there is at one point a blunder:

git clone https://github.com/bitcoin/bitcoin

should instead be:

git clone https://github.com/bitcoin/bitcoin
cd bitcoin
git fetch --tags
git checkout v$VERSION
cd ..

Otherwise you cannot build the correct version because you will be picking the .yml descriptors of master, whatever they are.

Example of failure:

debian@debian:~/gitian-builder$ ./bin/gbuild ../bitcoin/contrib/gitian-descriptors/deps-linux.yml
sha256sum: miniupnpc-1.9.tar.gz: No such file or directory
--- Building for precise i386 ---
Stopping target if it is up
Making a new image copy
stdin: is not a tty
Starting target
Checking if target is up
lxc-start: Connection refused - inotify event with no name (mask 32768)
Preparing build environment
lxc-start: Connection refused - inotify event with no name (mask 32768)
tar: miniupnpc-1.9.tar.gz: Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors
Updating apt-get repository (log in var/install.log)
Installing additional packages (log in var/install.log)
Grabbing package manifest
stdin: is not a tty
Creating build script (var/build-script)
Running build script (log in var/build.log)
./bin/gbuild:21:in `system!': failed to run on-target setarch i386 bash -x < var/build-script > var/build.log 2>&1 (RuntimeError)
    from ./bin/gbuild:116:in `build_one_configuration'
    from ./bin/gbuild:233:in `block (2 levels) in <main>'
    from ./bin/gbuild:228:in `each'
    from ./bin/gbuild:228:in `block in <main>'
    from ./bin/gbuild:226:in `each'
    from ./bin/gbuild:226:in `<main>'

I was building 0.9.1 and it tried to use descriptors from master. Note also as it didn't stop at the missing miniupnp library (submitted issue at gitian-builder project)

Yes, you should usually make sure that you use the descriptors from the version that you are using, although there can be reasons not to, for example when testing new descriptors against a fixed version.

ACK with mentioning this in the document.

this was fixed with #4279