sdiff: fix the layout of unchanged rows, blank lines and cut tabs

[sami-daniel]

src/side_diff.rs pads the left half up to a fixed column and writes the right half after it, instead of padding from the column where the text ended. Three rows come out different from GNU diff:

• The right half of an unchanged row starts at half_width + 3 instead of column two. The two are the same at the default --width=130, so it only shows at other widths and with --expand-tabs.
• A blank line common to both files is padded up to column two instead of left empty.
• A tab that reaches the end of the half is dropped, and the text after it is still printed. GNU diff cuts the row at that tab.

Tabs are shown as ^I and line ends as $:

$ diff -y --width=40 L R | cat -A      # L: a,(blank),b,c   R: a,(blank),c
a^I^I   a$
^I^I   $
b^I^I   <$
c^I^I   c$

$ diff -y --width=10 L R | cat -A      # L: a<TAB>b,c   R: c
ab  <$
c    c$

GNU diff 3.10 prints:

a^I^I^Ia$
$
b^I^I   <$
c^I^I^Ic$

a   <$
c^Ic$

So the change is confined to process_half_line and push_output. Print only the text of each half, keep the column where the output stopped, and pad from that column to the marker and to column two.