Add checkout strategy to stash apply/pop methods

[slavikus]

For a finer control over the unstash process, propogate checkout strategy to Objective-Git from the underlying libgit2 stash methods by adding an extra strategy argument to -[GTRepository applyStashAtIndex:…] and -[GTRepository popStashAtIndex:…]

[slavikus]

@tiennou , I'm going to flood you with these one by one now :D

[pietbrauer]

@slavikus Could you rebase this branch onto master with #459 being merged?

[slavikus]

@pietbrauer will do.

[slavikus]

@pietbrauer there we go.

[pietbrauer]

@slavikus So it's basically adding (with a default checkout strategy) and two new lines or did something go wrong in the merging?

[slavikus]

@pietbrauer In this PR, I've added checkout strategy as a separate parameter; in #459 , it was replaced by more generic (and more broad) GTCheckoutOptions class, so I've removed the more shallow variant that was previously used here.

[pietbrauer]

Ok, so something went wrong. There are only 4 changed lines

[slavikus]

Nah, not really. #459 basically superceeded this one entirely. :)

[slavikus]

However, now when I think of it, the "(with a default checkout strategy)" makes no sense anymore. :) I'll push a clarifying commit.

[pietbrauer]

Ok, cool. Thanks. No need to wait for CI then.