Remove 3-argument getbufvar() for better compatibility

References #95.
Prevent potential regressions

References #95.
Perforce: make it use g:signify_diffoptions

References #89.
Use 1-based indexing for buffer ranges

`:SyDebug` is displaying info if an alternate buffer is set.  This
occurs because a buffer number of 0 represents the alternate buffer.
Using a 1-based range in sy#debug#list_active_buffers fixes this.

Signed-off-by: James McCoy <vega.james@gmail.com>

References #97.