(Auto-)update patch

[ben-edna]

When using dfetch update it would be nice to have the possibility to auto-update the patch.
Because sometimes the patch fails because line nrs don't match.

Otherwise a cleaner procedure with user interaction to pull in the remote version and restore the changes applied previously. Something along the lines of:

• if there are local changes
• User runs dfetch update --some-flag MyModule
• dfetch stores current state of module
• dfetch force updates the module (in some tmp repo?)
• dfetch restores the old state of the tmp folder
• dfetch generates new patch at patch location