The code is updated to check the result of memory allocation before
it is used in more places, by using xmalloc and/or xcalloc calls.

* jk/xmalloc:
  progress: use xmalloc/xcalloc
  xdiff: use xmalloc/xrealloc
  xdiff: use git-compat-util
  test-prio-queue: use xmalloc