build: add make-archive.sh helper

Author: jpgrayson

contrib/release/README

@@ -25,8 +25,7 @@ StGit Release Checklist
 
 4. Make source archive
 
-   - `git archive --format=tar.gz --prefix=stgit-X.Y.Z/ -o stgit-X.Y.Z.tar.gz vX.Y.Z`
-   - Note the trailing slash in the `--prefix` value.
+   - `./contrib/releases/make-archive.sh`
    - This archive will be uploaded in step 7.
 
 5. Update stacked-git.github.io

contrib/release/make-archive.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+set -eu
+
+OUTPUT_DIR=${1:-.}
+TAG=$(git describe --match 'v*.*.*' --exact-match)
+VER=${TAG#v}
+ARCHIVE="stgit-${VER}.tar.gz"
+
+git archive --format=tar.gz --prefix="stgit-${VER}/" -o "${OUTPUT_DIR}/${ARCHIVE}" "${TAG}"
+
+printf "%s/%s\n" "${OUTPUT_DIR}" "${ARCHIVE}"