Override custom format.pretty for git-show(1)

When using git-show(1) be careful to specify the --pretty=medium option
so that a custom format.pretty git-config(1) setting does not affect the
output.

This fixes cover letter recall which relies on the git-show(1) format to
find the end of the tag contents.

Reported-by: Charlie Shepherd <charlie@ctshepherd.com>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>