Add Rebase option to Pull

[vvekic]

Just a simple addition of the --rebase flag for git pull.

[lunny]

The real problem is how to test it. But it seems it's a long plan to work. Currently LGTM.

[vvekic]

It is tested no worse than the rest of the package, basically.
I might delve into making tests that create local repos, commits and so on, but as the implementation relies entirely on the system "git" command, I see no point in doing so. The command can be expected to work flawlessly, and so can the extra "--rebase" flag.

[tboerger]

LGTM