user-manual: reorganize fetch discussion, add internals, etc.

user-manual: reorganize fetch discussion, add internals, etc.

Keep git remote discussion in the first chapter, but postpone
lower-level git fetch usage (to fetch individual branches) till later.

Import a bunch of slightly modified text from the readme to give an
architectural overview at the end.

Add more discussion of history rewriting.

And a bunch of other miscellaneous changes....

Signed-off-by: "J. Bruce Fields" <redacted>