Fix error reporting for git checkout of original rev

[jonmeow]

I noticed this due to problems with MODULE.bazel.lock in bazel 7.

Example failure: https://github.com/carbon-language/carbon-lang/actions/runs/7250904398/job/19752117231?pr=3514

2023/12/18 16:30:10 failed to process change: failed to check out original commit during cleanup: <nil>

With this change it should look more like:

2023/12/18 08:51:19 failed to process change: failed to check out original commit during cleanup: failed to check out revision 'after' (llvm-bzlmod, sha: 67bcc1f551049d5f29f23d33598f0eda73a23151): exit status 1. Output: error: Your local changes to the following files would be overwritten by checkout:
	MODULE.bazel.lock
Please commit your changes or stash them before you switch branches.

[illicitonion]

Thanks!