Fix pulling into the same branch.

Fix pulling into the same branch.

When the "git pull" command updates the branch head you are
currently on, before doing anything else, first update your
index file and the working tree contents to that of the new
branch head.  Otherwise, the later resolving steps would think
your index file is attempting to revert the change between the
original head commit and the updated head commit.

It uses two-tree fast-forward form of "read-tree -m -u" to
prevent losing whatever local changes you may have in the
working tree to do this update.  I think this would at least
make things safer (a lot safer), and prevent mistakes.

Also "git fetch" command is forbidden from fetching and fast
forwarding the current branch head unless --update-head-ok flag
is given.  "git pull" passes the flag when it internally calls
"git fetch".

Signed-off-by: Junio C Hamano <junkio@cox.net>