viewdiff: don't increment the reported hunk line number

viewdiff: don't increment the reported hunk line number

For a diff hunk starting at line N, diff_hunk() constructs the link
with "#n(N + 1)".  This sends the viewer one line below the first
context line.  Although this is minor and may not even be noticed,
there's not an obvious reason to increment the line number, so switch
to using the reported value as is.