fail pull/merge early in the middle of conflicted merge

fail pull/merge early in the middle of conflicted merge

After a pull that results in a conflicted merge, a new user
often tries another "git pull" in desperation.  When the index
is unmerged, merge backends correctly bail out without touching
either index nor the working tree, so this does not make the
wound any worse.

The user will however see several lines of messsages during this
process, such as "filename: needs merge", "you need to resolve
your current index first", "Merging...", and "Entry ... would be
overwritten by merge. Cannot merge.".  They are unnecessarily
alarming, and cause useful conflict messages from the first pull
scroll off the top of the terminal.

This changes pull and merge to run "git-ls-files -u" upfront and
stop them much earlier than we currently do.  Old timers may
know better and would not to try pulling again before cleaning
things up; this change adds extra overhead that is unnecessary
for them.  But this would be worth paying for to save new people
from needless confusion.

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