Clarify tree object example not a continuation of previous examples

[excalamus]

• I provide my work under the project license.
• I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

Alert readers who are following along in the shell that the next example does not use the repository that was just created.

Context

Section 10.2: Git Objects begins with creating a new repository and explaining how blobs are added to it. The active voice used throughout that section implies it is expected for readers to be following along within their own shell. The next section, 10.2: Tree Objects, begins with an example which is not related to the previous examples. The current wording is "most recent tree in a project may look something like this". However, the project created in the previous section is a not a valid interpretation of "a project"; it does not yet have a tree. As a result, readers may enter the command given into their shell and produce an unexpected result:

git cat-file -p master^^{tree}
fatal: Not a valid object name master^{tree}

The topic has just been introduced, so the reader has no understanding from which to make sense of the error. Was it something they did? Is it something else? There is no way for them to know without already understanding the section.

Since the section then continues to use the previously created repository, it seems appropriate to alert readers of the exception in the first example.

This problem has been encountered previously in #421 (fixes #421)

[ben]

✨ Thanks!