mfc-candidates.lua: add -F fmt option

Add -F fmt option to give a free format string to git show --pretty=.
It can be used, for example, as:

./tools/tools/git/mfc-candidates.lua -F "%h %cD %s"

to see which commits have passed the minimum 3 days of waiting before
MFCing, or to see which group of commits belong together.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D53172

(cherry picked from commit af445e1dd95d43c011f377ea6e6618637fecc61c)