While looping over multiple heads, an up-to-date head will clobber the `remote->need_pack` setting, preventing the rest of the machinery from building and downloading a pack-file, breaking fetches.
This changes the behavior of callbacks so that the callback error code is not converted into GIT_EUSER and instead we propagate the return value through to the caller. Instead of using the giterr_capture and giterr_restore functions, we now rely on all functions to pass back the return value from a callback. To avoid having a return value with no error message, the user can call the public giterr_set_str or some such function to set an error message. There is a new helper 'giterr_set_callback' that functions can invoke after making a callback which ensures that some error message was set in case the callback did not set one. In places where the sign of the callback return value is meaningful (e.g. positive to skip, negative to abort), only the negative values are returned back to the caller, obviously, since the other values allow for continuing the loop. The hardest parts of this were in the checkout code where positive return values were overloaded as meaningful values for checkout. I fixed this by adding an output parameter to many of the internal checkout functions and removing the overload. This added some code, but it is probably a better implementation. There is some funkiness in the network code where user provided callbacks could be returning a positive or a negative value and we want to rely on that to cancel the loop. There are still a couple places where an user error might get turned into GIT_EUSER there, I think, though none exercised by the tests.
The callback-based method of listing remote references dates back to the beginning of the network code's lifetime, when we didn't know any better. We need to keep the list around for update_tips() after disconnect() so let's make use of this to simply give the user a pointer to the array so they can write straightforward code instead of having to go through a callback.
This allows us to add e.g. "HEAD" as a refspec when none are given without overwriting the user's data.
The correct behaviour when a remote has no refspecs (e.g. a URL from the command-line) is to download the remote's HEAD. Let's do that. This fixes #1261.
The text progress and update_tips callbacks are already part of the struct, which was meant to unify the callback setup, but the download one was left out.
This is a significant reorganization of the diff code to break it into a set of more clearly distinct files and to document the new organization. Hopefully this will make the diff code easier to understand and to extend. This adds a new `git_diff_driver` object that looks of diff driver information from the attributes and the config so that things like function content in diff headers can be provided. The full driver spec is not implemented in the commit - this is focused on the reorganization of the code and putting the driver hooks in place. This also removes a few #includes from src/repository.h that were overbroad, but as a result required extra #includes in a variety of places since including src/repository.h no longer results in pulling in the whole world.
When tagopt is set to '--tags', we should only take the default tags refspec into account and ignore any configured ones. Bring the code into compliance.