Rewrite rebase vs merge analogy

[HonkingGoose]

A reader opened issue #351, saying that the rebase vs merge analogy used was not clear.
I tried my hand at rewriting this part. I hope this is better than what we had before.

• Add a sentence reminding readers that they themselves will benefit from a clean history as well. It's not just for others that a clean history is useful...
• Add a sentence explaining that messy work is cleaned before merging into the mainline branch.
• Remove the part that says: "and the manual for how to maintain your software deserves careful editing", as this is nonsensical in the context of how software is delivered to the end-users. I hope no sane company allows a developer to tell a end-user: "The manual are the Git commits, just install Git, figure out how to use git clone to get the history, and figure out how to use git log to read the history, then trawl trough 1000 commits, and find what you need to do." 😄

Merging this pull-request fixes #351.