Added --no-check-gitignore option for finer grained control than using --force.

Added --no-check-gitignore option for finer grained control than using --force.

add, addurl, importfeed, import: Added --no-check-gitignore option
for finer grained control than using --force.

(--force is used for too many different things, and at least one
of these also uses it for something else. I would like to reduce
--force's footprint until it only forces drops or a few other data
losses. For now, --force still disables checking ignores too.)

addunused: Don't check .gitignores when adding files. This is a behavior
change, but I justify it by analogy with git add of a gitignored file
adding it, asking to add all unused files back should add them all back,
not skip some. The old behavior was surprising.

In Command.Lock and Command.ReKey, CheckGitIgnore False does not change
behavior, it only makes explicit what is done. Since these commands are run
on annexed files, the file is already checked into git, so git add won't
check ignores.