Cleanups following gvfs-helper

[derrickstolee]

Here are a few leftovers from #71

• There were a few more "readobject" strings in the codebase.

• The PackfileMaintenanceStepTests were disabled, but now they are re-enabled and adjusted to the new gvfs-helper environment.

• The PrefetchVerb no longer does anything but the commits and trees prefetch, so drop the --commits option.

[wilbaker]

@derrickstolee unrelated to these specific changes, I saw that this change:

It's not! I thought it would speed up the test, but in fact it makes the test do nothing of value. I'll remove this and try the speed-up later (while also making these enlistments sparse).

I'll drop it from this PR.

Did end up getting merged in as part of #122, was that intentional? If not, should it get backed out in this PR?

[derrickstolee]

It's not! I thought it would speed up the test, but in fact it makes the test do nothing of value. I'll remove this and try the speed-up later (while also making these enlistments sparse).

I'll drop it from this PR.

Did end up getting merged in as part of #122, was that intentional? If not, should it get backed out in this PR?

Thanks for catching that, @wilbaker! I definitely thought I had changed that, but must have forgotten. I'll get it in this one.

[derrickstolee]

I removed the test change as it broke the build. I filed #156 to track fixing that test.