Problem: The session file does not restore the alternate file.
Solution: Add ":balt". Works like ":badd" and also sets the buffer as the
alternate file. Use it in the session file. (closes#7269,
closes#6714)
Problem: Still using default option values after using ":badd +1".
Solution: Find a window where options were set. Don't set the window when
using ":badd".
Problem: Default option values are changed when using :badd for an existing
buffer.
Solution: When calling buflist_new() pass a zero line number. (closes#7195)
Problem: assert_fails() checks the last error message.
Solution: Check the first error, it is more relevant. Fix all the tests
that rely on the old behavior.
Problem: Insufficient tests for src/buffer.c.
Solution: Add more tests. Move comments related tests to a separate file.
(Yegappan Lakshmanan, closes#6325)
