Partial history rewrites were possible before with the (previously
hidden) --refs flag, but the defaults were wrong. That could be worked
around with the --source or --target flags, but that disabled --no-data
for fast-export and thus slowed things down, and also would require
overridding --replace-refs. And the defaults for --source and --target
may diverge further from what is wanted/needed for partial history
rewrites in the future.
So, add --partial as a first-class supported option with scary
documentation about how it permits mixing new and old history. Make
--refs imply that flag. Make the behavioral similarities (in regards to
which steps are skipped) between --source, --target, and --partial more
clear. Add relevant documentation to round it out.
Signed-off-by: Elijah Newren <newren@gmail.com>
