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[PATCH 0/4] use xmalloc in more places

It was reported on the Git security list that there are a few spots
which use a bare malloc() but don't check the return value, which could
dereference NULL. I don't think any of these are exploitable in an
interesting way, beyond Git just segfaulting more or less immediately.

But we should still be handling failures, and I think it makes sense to
be consistent about how we do it (and the other rules which come with
using xmalloc, like GIT_ALLOC_LIMIT).

This series cleans up most of the bare calls found by:

  git grep -E '[^a-z_](m|c|re)alloc\(' '*.c' :^compat :^contrib :^wrapper.c

The calls I've left are:

  - wrapper.c obviously needs to call the real functions :)

  - compat/ has functions emulating libc and system calls, and which are
    expected to return ENOMEM as appropriate

  - diff-delta will gracefully return NULL when trying to delta
    something too large, and pack-objects will skip past trying to find
    a delta. I've never seen this happen in practice, but then I
    primarily use Linux which is more than happy to overcommit on
    malloc(). I've left it unchanged, though possibly we could have an
    xmalloc_gently() if we want to enforce things like GIT_ALLOC_LIMIT
    but still do the graceful fallback thing.

  - test-hash tries to probe malloc() to see how big a buffer it can
    allocate. I doubt this even does anything useful on systems like
    Linux that overcommit. We also don't seem to ever invoke this with a
    buffer larger than 8k in the first place. So it could maybe go away
    entirely, but I left it here.

  [1/4]: test-prio-queue: use xmalloc
  [2/4]: xdiff: use git-compat-util
  [3/4]: xdiff: use xmalloc/xrealloc
  [4/4]: progress: use xmalloc/xcalloc

 progress.c                 | 18 +++++-------------
 t/helper/test-prio-queue.c |  2 +-
 xdiff/xdiff.h              |  4 ++--
 xdiff/xinclude.h           |  8 +-------
 4 files changed, 9 insertions(+), 23 deletions(-)