gitweb: Extract print_sidebyside_diff_lines()

gitweb: Extract print_sidebyside_diff_lines()

Currently, print_sidebyside_diff_chunk() does two things: it
accumulates diff lines and prints them.  Accumulation may be used to
perform additional operations on diff lines, so it makes sense to split
these two things.  Thus, whole code that formats and prints diff lines
in the 'side-by-side' manner is moved out of print_sidebyside_diff_chunk()
to a separate subroutine and two conditions that control printing
diff liens are merged.

Thanks to that, we can easily (in later patches) replace call to that
subroutine with a call to more generic print_diff_lines() that will
control whether 'inline' or 'side-by-side' diff should be printed.

As a side effect, context lines are printed just before printing added
and removed lines, and at the end of chunk (previously, they were
printed immediately on the class change).  However, this doesn't change
gitweb output.

The outcome of this patch is that print_sidebyside_diff_chunk() is now
much shorter and easier to read.

While at it, drop the '# assume that it is change' comment.  According
to Jakub Narębski:

	What I meant here when I was writing it that they are lines that
	changed between two versions, like '!' in original (not unified)
	context format.

	We can omit this comment.

Signed-off-by: Michał Kiedrowicz <redacted>
Acked-by: Jakub Narębski <redacted>
Signed-off-by: Junio C Hamano <redacted>