Updating to the latest version / rolling back to a working one

[Ross Smith <myxiplx googlemail com>]

Hi folks,

Could I ask for some help for a git newbie?  I tried to get the latest
version yesterday, but gnome-shell just kept crashing, which after
lurking on the IRC channel, I guess is a bug in the current build.

Could I ask:
- Does running jhbuild download and compile the latest code each time,
or does it just re-compile what I have?
- Do I need to run git fetch or something to get updates?
- Can I roll back to an earlier commit if the latest is broken?  Is
"git log --summary" the best way to see where to go back to?
- I used these instructions to install breadcrumbs:
http://mail.gnome.org/archives/gnome-shell-list/2009-May/msg00037.html
  But I couldn't revert back to master, instead I had to use "git
checkout master -f".  Is that normal?

And finally, the big one:
- Is there any danger of me breaking the repository?  I really hope I
have read only access.

thanks,

Ross