[Bug?] log -p -W showing the whole file for a patch that adds to the end?

Just noticed a curiosity:

    $ git show -W 3e3ceaa58 quote.c

shows the entire file.  The commit in question adds a whole new
function at the end of the file.  If I move that addition to just
before the last function the file already had before the change and
amend the commit, "show -W" would work as expected, i.e. addition of
the function, with three lines before and three lines after context.

The -W feature was added by 14937c2c (diff: add option to show whole
functions as context, 2011-10-09).