apply: count the size of postimage correctly

apply: count the size of postimage correctly

Under --whitespace=fix option, match_fragment() function examines
the preimage (the common context and the removed lines in the patch)
and the file being patched and checks if they match after correcting
all whitespace errors.  When they are found to match, the common
context lines in the preimage is replaced with the fixed copy,
because these lines will then be copied to the corresponding place
in the postimage by a later call to update_pre_post_images().  Lines
that are added in the postimage, under --whitespace=fix, have their
whitespace errors already fixed when apply_one_fragment() prepares
the preimage and the postimage, so in the end, application of the
patch can be done by replacing the block of text in the file being
patched that matched the preimage with what is in the postimage that
was updated by update_pre_post_images().

In the earlier days, fixing whitespace errors always resulted in
reduction of size, either collapsing runs of spaces in the indent to
a tab or removing the trailing whitespaces.  These days, however,
some whitespace error fix results in extending the size.

250b3c6c (apply --whitespace=fix: avoid running over the postimage
buffer, 2013-03-22) tried to compute the final postimage size but
its math was flawed.  It counted the size of the block of text in
the original being patched after fixing the whitespace errors on its
lines that correspond to the preimage.  That number does not have
much to do with how big the final postimage would be.

Instead count (1) the added lines in the postimage, whose size is
the same as in the final patch result because their whitespace
errors have already been corrected, and (2) the fixed size of the
lines that are common.

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