user-manual: clarify language about "modifying" old commits
user-manual: clarify language about "modifying" old commits

It's important to remember that git doesn't really allowing "editing" or
"modifying" commits, only replacing them by new commits.  Redo some of
the language to make this clearer.

Signed-off-by: J. Bruce Fields <51738506c1b2ccb0761f23bdc612c93babf738ea@citi.umich.edu>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git