mfc-candidates.lua: add -F fmt option

[bz]

Summary

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 -X exclude-list-15 -F "%h %cD %s"
to see which commits have passed the minimum 3 days of waiting before
MFCing, ot to see which group of commits belong together.

MFC after: 3 days

Test Plan

A take add adding the option to specify the git show format.

[bz]

This comment was removed by bz.

[emaste]

LGTM with one pedantic nit

tools/tools/git/mfc-candidates.lua

120

I guess we should sort the options. We have examples in the tree already for both -F -f -t -u -X (alpha order, uppercase first) and -F -X -f -t -u

[bz]

I do not like either option particularly well as I would have
preferred to keep the "important" options first but this seems
less intrusive.