Remove "git pull" called during "convert"

I don't think we should call "git pull" during "convert" if the work
directory exists, only do a reset. The reason is that "pull" can change
the source code you are working with under your hands, which is really
bad during debugging.

If needed, we can add "yk pull" command to invoke "git pull" manually.
Merge pull request #247 from yast/no-pull-during-convert

Remove "git pull" called during "convert"