Move the delta application functions into `delta.c`, next to the similar delta creation functions. Make the `git__delta_apply` functions adhere to other naming and parameter style within the library.
When looking up an abbreviated oid, show the actual (abbreviated) oid the caller passed instead of a full (but ambiguously truncated) oid.
In the current implementation of ODB backends, each backend is tasked with refreshing itself after a failed lookup. This is standard Git behavior: we want to e.g. reload the packfiles on disk in case they have changed and that's the reason we can't find the object we're looking for. This behavior, however, becomes pathological in repositories where multiple alternates have been loaded. Given that each alternate counts as a separate backend, a miss in the main repository (which can potentially be very frequent in cases where object storage comes from the alternate) will result in refreshing all its packfiles before we move on to the alternate backend where the object will most likely be found. To fix this, the code in `odb.c` has been refactored as to perform the refresh of all the backends externally, once we've verified that the object is nowhere to be found. If the refresh is successful, we then perform the lookup sequentially through all the backends, skipping the ones that we know for sure weren't refreshed (because they have no refresh API). The on-disk pack backend has been adjusted accordingly: it no longer performs refreshes internally.
Coverity complains about the git_rawobj ones because we use a loop in which we keep remembering the old version, and we end up copying our object as the base, so we want to have the data pointer be NULL.
Opening the same repository multiple times will currently open the same file multiple times, as well as map the same region of the file multiple times. This is not necessary, as the packfile data is immutable. Instead of opening and closing packfiles directly, introduce an indirection and allocate packfiles globally. This does mean locking on each packfile open, but we already use this lock for the global mwindow list so it doesn't introduce a new contention point.
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.
This adds giterr_user_cancel to return GIT_EUSER and clear any error message that is sitting around. As a result of using that in places, we need to be more thorough with capturing errors that happen inside a callback when used internally. To help with that, this also adds giterr_capture and giterr_restore so that when we internally use a foreach-type function that clears errors and converts them to GIT_EUSER, it is easier to restore not just the return value, but the actual error message text.