This makes it easier to pass connection-related options around (proxy & custom headers for now). This fixes a bug in git_push_finish, which didn't reuse the provided proxy if the connection closed between the call to `git_remote_push` and the finish step.
Next to including several files, our "common.h" header also declares various macros which are then used throughout the project. As such, we have to make sure to always include this file first in all implementation files. Otherwise, we might encounter problems or even silent behavioural differences due to macros or defines not being defined as they should be. So in fact, our header and implementation files should make sure to always include "common.h" first. This commit does so by establishing a common include pattern. Header files inside of "src" will now always include "common.h" as its first other file, separated by a newline from all the other includes to make it stand out as special. There are two cases for the implementation files. If they do have a matching header file, they will always include this one first, leading to "common.h" being transitively included as first file. If they do not have a matching header file, they instead include "common.h" as first file themselves. This fixes the outlined problems and will become our standard practice for header and source files inside of the "src/" from now on.
While this will rarely be different from the default, having it in the remote adds yet another setting it has to keep around and can affect its behaviour. Move it to the options.
Instead of having it set in a different place from every other callback, put it the main structure. This removes some state from the remote and makes it behave more like clone, where the constructors are passed via the options.
Having the setting be different from calling its actions was not a great idea and made for the sake of the wrong convenience. Instead of that, accept either fetch options, push options or the callbacks when dealing with the remote. The fetch options are currently only the callbacks, but more options will be moved from setters and getters on the remote to the options. This does mean passing the same struct along the different functions but the typical use-case will only call git_remote_fetch() or git_remote_push() and so won't notice much difference.
Remote-tracking branch prunning
Instead we provide git_remote_upload() and git_remote_update_tips() in order to have a parallel API for fetching and pushing.
When a list of refspecs is passed to fetch (what git would consider refspec passed on the command-line), we not only need to perform the updates described in that refspec, but also update the remote-tracking branch of the fetched remote heads according to the remote's configured refspecs. These "fetches" are not however to be written to FETCH_HEAD as they would be duplicate data, and it's not what the user asked for.
The configured/base fetch refspecs need to be taken into account in order to implement opportunistic remote-tracking branch updates. DWIM them and store them in the struct, but don't do anything with them yet.