With `fetch: if_needed`, rbm is doing a git fetch when it shouldn't, when using a fixed commit

With 8c773150 we're checking if the output of git rev-parse is the same as git_hash to see if we can avoid doing a fetch (when git_hash is a fixed commit).

However the output of git rev-parse contains a newline, which we don't take into account when doing the comparison, resulting in the check always failing, and a git fetch being run when it shouldn't.

/cc @pierov