Makefile: refactor generators to be PWD-independent

We have multiple scripts that generate headers from other data. All of
these scripts have the assumption built-in that they are executed in the
current source directory, which makes them a bit unwieldy to use during
out-of-tree builds.

Refactor them to instead take the source directory as well as the output
file as arguments.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

generate-hooklist.sh

@@ -2,6 +2,17 @@
 #
 # Usage: ./generate-hooklist.sh >hook-list.h
 
+SOURCE_DIR="$1"
+OUTPUT="$2"
+
+if test -z "$SOURCE_DIR" || ! test -d "$SOURCE_DIR" || test -z "$OUTPUT"
+then
+	echo >&2 "USAGE: $0 <SOURCE_DIR> <OUTPUT>"
+	exit 1
+fi
+
+{
+
 cat <<EOF
 /* Automatically generated by generate-hooklist.sh */
 
@@ -11,10 +22,12 @@ EOF
 sed -n \
 	-e '/^~~~~*$/ {x; s/^.*$/	"&",/; p;}' \
 	-e 'x' \
-	<Documentation/githooks.txt |
+	<"$SOURCE_DIR"/Documentation/githooks.txt |
 	LC_ALL=C sort
 
 cat <<EOF
 	NULL,
 };
 EOF
+
+} >"$OUTPUT"