Makefile.in: simplify logic for CFG_VER_HASH

git log -1 --pretty=format:'%H' is a very convoluted way of saying git
rev-parse HEAD.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
auto merge of #6951 : artagnon/rust/makefile-git, r=brson

git log -1 --pretty=format:'%H' is a very convoluted way of saying git
rev-parse HEAD.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>