[binutils-gdb] tui-winsource: Allocate for actual lines only

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 7bc2c8b83ea82b4315c67e7658af815aed062e73 ***

Author: Andreas Arnez <arnez@linux.vnet.ibm.com>
Branch: master
Commit: 7bc2c8b83ea82b4315c67e7658af815aed062e73

tui-winsource: Allocate for actual lines only

The logic for allocating a TUI source window's content buffer allocates
two more lines than needed, because it does not reduce the window height
by the highlight box's overhead.  However, it does reduce the line width
accordingly.  This patch makes the height and width calculation
consistent and improves the comment.

gdb/ChangeLog:

	* tui/tui-winsource.c (tui_alloc_source_buffer): Subtract
	highlight box's overhead when calculating the content height.