At some level I am more or less indifferent, in the sense that both
compat-util and common need to be included by a .c file anyway,
whether it is _our_ source, or in a future third-party source that
wants to link with the libified part of Git, so it does notreally
make much of a difference if ARRAY_SIZE() is defined in which one of
these two headers.  But presumably such a future third-party source
may arrange their own system support using their own counterpart to
compat-util without using ours, and if that is what motivates this
change, I could understand why a more-or-less git-agnostic common.h
could be a good idea.

Also, a later patch moves ALLOC_GROW and other macros from alloc.h to
common.h. I thought it would make more sense to have a common file for
these macros to be moved to rather than adding to git-compat-util for
the sake of only removing the alloc.h dependency from certain files.

Everything, other than sane-ctype macros, does look like a generally
useful git-agnostic helper functions.  If we were to move in this
direction, I probably would prefer to separate sane-ctype macros in
a separate header, simply because they are _designed_ to override
system supplied ones and some third-party source may not want to use
such a "feature".

Ah that makes sense that they would belong in a separate file like
ctype.h -- I can split those macros out.