Move zfs_gitrev.h to build directory

Currently an out-of-tree build does not work with read-only source
directory because zfs_gitrev.h can't be created. Move this file to the
build directory, which is more appropriate for a generated file, and
drop the dist-hook for zfs_gitrev.h. There is no need to distribute this
file since it will be regenerated as part of the compilation in any
case.

scripts/make_gitrev.sh tries to avoid updating zfs_gitrev.h if there has
been no change, however this doesn't cover the case when the source
directory is not in git: in that case zfs_gitrev.h gets overwritten even
though it's always "unknown". Simplify the logic to always write out a
new version of zfs_gitrev.h, compare against the old and overwrite only
if different. This is now simple enough to just include in the
Makefile, so drop the script.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10493

scripts/Makefile.am

@@ -15,7 +15,6 @@ EXTRA_DIST = \
 	dkms.postbuild \
 	enum-extract.pl \
 	kmodtool \
-	make_gitrev.sh \
 	man-dates.sh \
 	paxcheck.sh \
 	zfs2zol-patch.sed \

scripts/make_gitrev.sh

@@ -1,45 +0,0 @@
-#!/bin/sh
-
-#
-# CDDL HEADER START
-#
-# This file and its contents are supplied under the terms of the
-# Common Development and Distribution License ("CDDL"), version 1.0.
-# You may only use this file in accordance with the terms of version
-# 1.0 of the CDDL.
-#
-# A full copy of the text of the CDDL should have accompanied this
-# source.  A copy of the CDDL is also available via the Internet at
-# http://www.illumos.org/license/CDDL.
-#
-# CDDL HEADER END
-#
-
-# Copyright (c) 2018 by Delphix. All rights reserved.
-# Copyright (c) 2018 by Matthew Thode. All rights reserved.
-
-#
-# Generate zfs_gitrev.h.  Note that we need to do this for every
-# invocation of `make`, including for incremental builds.  Therefore we
-# can't use a zfs_gitrev.h.in file which would be processed only when
-# `configure` is run.
-#
-
-set -e -u
-
-cleanup() {
-    ZFS_GIT_REV=${ZFS_GIT_REV:-"unknown"}
-    cat << EOF > "$(dirname "$0")"/../include/zfs_gitrev.h
-#define	ZFS_META_GITREV "${ZFS_GIT_REV}"
-EOF
-}
-trap cleanup EXIT
-
-# Check if git is installed and we are in a git repo.
-git rev-parse --git-dir > /dev/null 2>&1
-# Get the git current git revision
-ZFS_GIT_REV=$(git describe --always --long --dirty 2>/dev/null)
-# Check if header file already contain the exact string
-grep -sq "\"${ZFS_GIT_REV}\"" "$(dirname "$0")"/../include/zfs_gitrev.h &&
-	trap - EXIT
-exit 0