repository: move fetch_if_missing into struct repository

[malon7782]

The global variable 'fetch_if_missing' controls whether a missing object check should prompt a lazy fetch from a promisor remote. In order to continue the libification effort, Move it into 'struct repository' and initialize it to 1 by default to keep the previous behavior.

Subsystems that already pass around a repository pointer, are updated to read this flag directly from their respective 'repo' instances. Builtin commands are updated to modify
'the_repository->fetch_if_missing'. Where necessary, the 'USE_THE_REPOSITORY_VARIABLE' macro is defined to allow these commands to access 'the_repository'.

Additionally, update the partial clone documentation to reflect that this is now a per-repository flag.

Mentored-by: Christian Couder christian.couder@gmail.com
Mentored-by: Ayush Chandekar ayu.chandekar@gmail.com
Mentored-by: Olamide Caleb Bello belkid98@gmail.com

[malon7782]

I'm very unconvinced about the changes in this patch, although they are so simple and might be correct formally.

The reason is that I am still trying to figure out the mechanism of partial clone and promise remote (that's why I haven't explained what fetch_if_missing does in detail );
moreover, this variable is obviously different from the previous static configurations. It is a state variable that will be read and written frequently during runtime. Based on this, the only conclusion I can draw is that it is not so bad to put it in struct repository

[malon7782]

I'm very unconvinced about the changes in this patch, although they are so simple and might be correct formally.

The reason is that I am still trying to figure out the mechanism of partial clone and promise remote; moreover, this variable is obviously different from the previous static configurations. It is a state variable that will be read and written frequently during runtime. Based on this, the only conclusion I can draw is that it is not so bad to put it in struct repository

In other words, repository seems to be the least bad place to go. Because repo_settings, repo_config_values, raw_object_store, index_state are not intuitively appropriate. If we are sure to add it to the struct repository, we need to demonstrate that fetch_if_missing is a kill switch that is critical enough.