Turn the `fetch_if_missing` global into a field of `struct repository`

[dscho]

It is high time that we reduced our globals, especially in light of finally having struct repository and working towards using it everywhere.

Hello Git contributors
I am Shubham Ghule.I am an engineering student.I am new to an open source and I am excited to contribute to Git.
I have tired various options to solve #251 bug and came up with a solution.This patch will convert the extern integer 'fetch_if_missing' into a field of 'struct repository'.
I would really like to submit the patch.
I am looking forward to your review.
Thanks!
Shubham Ghule.
@dscho

[dscho]

@dev-shubham1 no need to ask for permission ;-)