git: clean up "fetch" method

De-duplicate repetitive code checking the exit code.

Include the stdout/stderr of the failed process in all cases.

Remove the returned values because no caller uses them.

Combine git commands where possible.  There is no need to fetch branches
and tags as two separate operations.
git: add 'refspec' argument

This argument may be used to fetch additional refs beyond the default
refs/heads/* and refs/tags/*.  Checking out GitHub pull requests or Gerrit
patch sets are two examples where this is useful.

Without this, specifying version=<sha1> with a SHA1 unreachable from any
tag or branch can't work.
Merge pull request #371 from rohanpm/git_refspec

git: add 'refspec' argument