Since writing multiple objects may all already exist in a single packfile, avoid freshening that packfile repeatedly in a tight loop. Instead, only freshen pack files every 2 seconds.
When writing an object, we calculate its OID and see if it exists in the object database. If it does, we need to freshen the file that contains it.
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.