Fix up git-merge-one-file-script

Junio points out that we may need to create the path leading
up the the file we merge.

And we need to be more careful with the "exec"s we've done
to exit on success - only do the on the last command in the
pipeline, not the first one ;)

git-merge-one-file-script

@@ -16,6 +16,14 @@
 # been handled already by git-read-tree, but that one doesn't
 # do any merges that might change the tree layout.
 
+verify_path() {
+    file="$1"
+    dir=`dirname "$file"` &&
+    mkdir -p "$dir" &&
+    rm -f -- "$file" &&
+    : >"$file"
+}
+
 case "${1:-.}${2:-.}${3:-.}" in
 #
 # Deleted in both.
@@ -32,8 +40,8 @@ case "${1:-.}${2:-.}${3:-.}" in
 #
 "$1.$1" | "$1$1.")
 	echo "Removing $4"
-	exec rm -f -- "$4" &&
-		git-update-cache --remove -- "$4"
+	rm -f -- "$4" &&
+		exec git-update-cache --remove -- "$4"
 	;;
 
 #
@@ -42,9 +50,10 @@ case "${1:-.}${2:-.}${3:-.}" in
 ".$2." | "..$3" )
 	case "$6$7" in *7??) mode=+x;; *) mode=-x;; esac
 	echo "Adding $4 with perm $mode."
-	exec git-cat-file blob "$2$3" >"$4" &&
+	verify_path "$4" &&
+		git-cat-file blob "$2$3" >"$4" &&
 		chmod $mode -- "$4" &&
-		git-update-cache --add -- "$4"
+		exec git-update-cache --add -- "$4"
 	;;
 
 #
@@ -58,9 +67,10 @@ case "${1:-.}${2:-.}${3:-.}" in
 	fi
 	case "$6" in *7??) mode=+x;; *) mode=-x;; esac
 	echo "Adding $4 with perm $mode"
-	exec git-cat-file blob "$2" >"$4" &&
+	verify_path "$4" &&
+		git-cat-file blob "$2" >"$4" &&
 		chmod $mode -- "$4" &&
-		git-update-cache --add -- "$4"
+		exec git-update-cache --add -- "$4"
 	;;
 
 #
@@ -72,7 +82,8 @@ case "${1:-.}${2:-.}${3:-.}" in
 	src1=`git-unpack-file $2`
 	src2=`git-unpack-file $3`
 
-	merge -p "$src1" "$orig" "$src2" > "$4"
+	verify_path "$4" &&
+		merge -p "$src1" "$orig" "$src2" > "$4"
 	ret=$?
 	rm -f -- "$orig" "$src1" "$src2"