Fix library update for git

[rpatel3001]

Fetch instead of pull so tags work in sync-version, and do it first so we get updates before checking out the version.

[rpatel3001]

Needs some work, this doesn't work for branches anymore

[olofk]

Just let me know when you have something I should review. I remember that there were some tricks done to correctly get tags and branches.

[rpatel3001]

not actively working on this but pasting a link for later
https://stackoverflow.com/a/18222877/2708313

[rpatel3001]

i don't know that anything fancy is really needed here, I reverted my previous change and just had it not print out the scary red error message if git returns non-zero. It still prints out the git output

You are not currently on a branch.
Please specify which branch you want to merge with.
See git-pull(1) for details.

    git pull <remote> <branch>

when sync-version is a tag or hash because the Launcher class is a pretty thin wrapper and you can't control that, but if a library is on a tag or commit maybe a user should just expect that. a library update for tags or commits should probably just be no-ops. It might be desirable to have Launcher return the process output and re-raise the RuntimeError if that text isn't in the output, just in case there are other reasons git fails and you want the scary red message back?