user-manual: describe how higher stages are set during a merge
user-manual: describe how higher stages are set during a merge

Higher stages store the blobs involved from their side verbatim.  Removal
of uninteresting hunks are done by "diff --cc" upon demand and not stored
in the index.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

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