Almost everything in what the patch is wrong.
* It creates <abspath.h> as a tracked file, but has a funny comment
with control-G in it that says "do not edit"
* It is entirely unclear how the contents of <abspath.h> is meant
to evolve, if the contributors are forbidden to edit it, and
without any instruction how to update it.
* The new <abspath.h> lost all function comments, grouping by
category, etc. that were in the original <cache.h>.
* There is an "#undef INTERFACE" that has no use to us, those who
develop Git, for no clearly explained reason.
* The proposed log message says what it did, but it does not even
try to justify why it is a good idea to do so.
* There is no need to create a new <abspath.h> in the first place.
If we were in an alternate world where we did not have
<abspath.c>, it may be reasonable to add <abspath.h> along with
it, instead of adding declarations for new functions and types in
<cache.h>. But the difference falls into "once it is in, it is
not worth the patch churn to change it" category.
The easiest way to make it correct is to drop it, I guess.
Thanks.