test-lib-functions: add and use a "test_hook" wrapper

Add a "test_hook" wrapper similar to the existing "test_config"
wrapper added in d960c47a88 (test-lib: add helper functions for
config, 2011-08-17).

This wrapper:

 - Will clean up the hook with "test_when_finished", unless --setup is
   provided.

 - Will error if we clobber a hook, unless --clobber is provided.

 - Takes a name like "update" instead of ".git/hooks/update".

 - Accepts -C <dir>, like "test_config" and "test_commit".

By using a wrapper we'll be able to easily change all the hook-related
code that assumes that the template-created ".git/hooks" directory is
created by "init", "clone" etc. once another topic follows-up and
changes the test suite to stop creating trash directories using those
templates.

In addition this will make it easy to have the hooks configured using
the "configuration-based hooks" topic, once we get around to
integrating that. I.e. we'll be able to run the tests in a mode where
we sometimes create a .git/hooks/<name>, and other times create a
script in another location, and point the relevant configuration
snippet to it.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -551,6 +551,58 @@ write_script () {
 	chmod +x "$1"
 }
 
+# Usage: test_hook [options] <hook-name> <<-\EOF
+#
+#   -C <dir>:
+#	Run all git commands in directory <dir>
+#   --setup
+#	Setup a hook for subsequent tests, i.e. don't remove it in a
+#	"test_when_finished"
+#   --clobber
+#	Overwrite an existing <hook-name>, if it exists. Implies
+#	--setup (i.e. the "test_when_finished" is assumed to have been
+#	set up already).
+test_hook () {
+	setup= &&
+	clobber= &&
+	indir= &&
+	while test $# != 0
+	do
+		case "$1" in
+		-C)
+			indir="$2" &&
+			shift
+			;;
+		--setup)
+			setup=t
+			;;
+		--clobber)
+			clobber=t
+			;;
+		-*)
+			BUG "invalid argument: $1"
+			;;
+		*)
+			break
+			;;
+		esac &&
+		shift
+	done &&
+
+	git_dir=$(git -C "$indir" rev-parse --absolute-git-dir) &&
+	hook_dir="$git_dir/hooks" &&
+	hook_file="$hook_dir/$1" &&
+	if test -z "$clobber"
+	then
+		test_path_is_missing "$hook_file"
+	fi &&
+	if test -z "$setup$clobber"
+	then
+		test_when_finished "rm \"$hook_file\""
+	fi &&
+	write_script "$hook_file"
+}
+
 # Use test_set_prereq to tell that a particular prerequisite is available.
 # The prerequisite can later be checked for in two ways:
 #