[lmi] Set working copy to a specific SHA1

[Greg Chicares]

Vadim--I think I've found a good solution to the problem below, but would
you handle it differently?

Kim maintains an lmi repository from which she prepares releases; she never
commits from it. She wants to release code from five commits ago:

git log --abbrev-commit --pretty=oneline
ae5c2ad Satisfy library dependencies for native builds
d90ded4 Do not use 'wine' to run native binaries
bac897e Prefer getchar() to getch()
fba8064 Install native toolchain in chroot
9c7870a Install full vim package in chroot and configure it
11ff9f0 Designate release candidate 20170411T2007Z

I suggested doing this in a fully updated local repository:

$git reset --hard 11ff9f0
$HEAD is now at 11ff9f0 Designate release candidate 20170411T2007Z

in order to prepare the release, and then later

$git pull

to resynchronize with HEAD of master.