Display a progress meter during merge-recursive.

Display a progress meter during merge-recursive.

Because large merges on slow systems can take up to a minute to
execute we should try to keep the user entertained with a progress
meter to let them know how far we have progressed through the
current merge.

The progress meter considers each entry in the in-memory index to
be a unit, which means a single recursive merge will double the
number of units in the progress meter.  Files which are unmerged
after the 3-way tree merge are also considered a unit within the
progress meter.

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