Add a "dirty" state to the index when it has unsaved changes

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.

When the index is dirty, return GIT_EINDEXDIRTY so that consumers can
identify the exact problem programatically.

Fix typo

Fix some type issues

More fixes

Address requested changes

Add test

Fix naming

Fix condition and tests

Address requested changes

Fix typo

Signed-off-by: Sven Strickroth <email@cs-ware.de>