Add option to disable --prune when updating repository

[k0rnpisey]

It would be grate to have an option to disable prune remote-tracking branches no longer on remote when updating a repositoty.

Snippet code of how it is implemented:

if lfs_clone:
    git_command = ['git', 'lfs', 'fetch', '--all', '--prune']
 else:
    git_command = ['git', 'fetch', '--all', '--force', '--tags', '--prune']

I'm willing to give PR for this option.

Do you think having this option interferes with others?