Merge branch 'jk/misc-uninitialized-fixes'

Various fixes to codepaths gcc 9 had trouble following dataflow.

* jk/misc-uninitialized-fixes:
pack-objects: drop packlist index_pos optimization
test-read-cache: drop namelen variable
diff-delta: set size out-parameter to 0 for NULL delta
bulk-checkin: zero-initialize hashfile_checkpoint
pack-objects: use object_id in packlist_alloc()
git-am: handle missing "author" when parsing commit

diff --cc builtin/pack-objects.c
Simple merge

diff --cc pack-bitmap.c
Simple merge