settings: optional unsaved index safety

Add the `GIT_OPT_ENABLE_UNSAVED_INDEX_SAFETY` option, which will cause
commands that reload the on-disk index to fail if the current
`git_index` has changed that have not been saved.  This will prevent
users from - for example - adding a file to the index then calling a
function like `git_checkout` and having that file be silently removed
from the index since it was re-read from disk.

Now calls that would re-read the index will fail if the index is
"dirty", meaning changes have been made to it but have not been written.
Users can either `git_index_read` to discard those changes explicitly,
or `git_index_write` to write them.

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