Improve merge performance by avoiding in-index merges.

In the early days of Git we performed a 3-way read-tree based merge
before attempting any specific merge strategy, as our core merge
strategies of merge-one-file and merge-recursive were slower script
based programs which took far longer to execute.  This was a good
performance optimization in the past, as most merges were able to
be handled strictly by `read-tree -m -u`.

However now that merge-recursive is a C based program which performs
a full 3-way read-tree before it starts running we need to pay the
cost of the 3-way read-tree twice if we have to do any sort of file
level merging.  This slows down some classes of simple merges which
`read-tree -m -u` could not handle but which merge-recursive does
automatically.

For a really trivial merge which can be handled entirely by
`read-tree -m -u`, skipping the read-tree and just going directly
into merge-recursive saves on average 50 ms on my PowerPC G4 system.
May sound odd, but it does appear to be true.

In a really simple merge which needs to use merge-recursive to handle
a file that was modified on both branches, skipping the read-tree
in git-merge saves on average almost 100 ms (on the same PowerPC G4)
as we avoid doing some work twice.

We only avoid `read-tree -m -u` if the only strategy to use is
merge-recursive, as not all merge strategies perform as well as
merge-recursive does.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

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