Add sharness based test suite

.github/workflows/shellcheck.yml

@@ -15,4 +15,4 @@ jobs:
       - name: Run ShellCheck
         uses: ludeeus/action-shellcheck@master
         with:
-            ignore_paths: ./completion.*
+            ignore_paths: ./completion.* ./t/sharness.sh

.github/workflows/tests.yml

@@ -0,0 +1,16 @@
+name: Tests
+
+on:
+  push:
+    branches: [ "master" ]
+  pull_request:
+    branches: [ "master" ]
+
+jobs:
+  test:
+    name: Sharness tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Run test suite
+        run: make test

.gitignore

@@ -0,0 +1 @@
+t/test-results/

Makefile

@@ -2,6 +2,13 @@ PREFIX?=/usr/local
 INSTALLDIR?=$(PREFIX)
 INSTALL=install
 
+TESTS?=$(wildcard t/t[0-9]*.sh)
+
+.PHONY: test install install-fish install-zsh
+
+test:
+	@prove $(TESTS)
+
 install:
 	${INSTALL} -d ${DESTDIR}${INSTALLDIR}/bin
 	${INSTALL} -m755 git-fixup ${DESTDIR}${INSTALLDIR}/bin/git-fixup

t/sharness.d/git-fixup.sh

@@ -0,0 +1,103 @@
+# Shared test helpers for the git-fixup test suite.
+#
+# This file is sourced automatically by sharness through t/sharness.d/.
+# shellcheck shell=sh
+# shellcheck disable=SC2154
+
+# Isolate the tests from the user's own git configuration.
+XDG_CONFIG_HOME="$SHARNESS_TRASH_DIRECTORY/.config"; export XDG_CONFIG_HOME
+# The user scope alone is not enough: /etc/gitconfig can carry settings
+# (init.defaultBranch, commit.gpgsign, ...) that alter test outcomes.
+GIT_CONFIG_NOSYSTEM=1; export GIT_CONFIG_NOSYSTEM
+# Editing should never block or spawn an interactive editor during tests.
+GIT_EDITOR=:; export GIT_EDITOR
+GIT_SEQUENCE_EDITOR=:; export GIT_SEQUENCE_EDITOR
+# Test repositories get a fixed identity from the environment, so tests do not
+# need per-repository git config for an author and committer (as in git/git).
+GIT_AUTHOR_NAME="Test User"; export GIT_AUTHOR_NAME
+GIT_AUTHOR_EMAIL=test@test.com; export GIT_AUTHOR_EMAIL
+GIT_COMMITTER_NAME="Test User"; export GIT_COMMITTER_NAME
+GIT_COMMITTER_EMAIL=test@test.com; export GIT_COMMITTER_EMAIL
+
+# Compare two files and print a unified diff when they differ. The first
+# argument is the expected content, the second the actual content, so the
+# labels in the diff line up with the git convention test_cmp expected actual.
+test_cmp () {
+	diff -u "$1" "$2"
+}
+
+# Write <content> to <file>, honouring printf escapes such as \n, and stage
+# the new content in the current repository.
+# Note: POSIX sh has no local, so parameters live in prefixed names to avoid
+# clobbering variables of the calling test body.
+stage_file () {
+	_file=$1
+	_content=$2
+	printf '%b' "$_content" >"$_file" &&
+	git add -- "$_file"
+}
+
+# Write <content> to <file> and commit it, like git/git's test_commit() but
+# without creating a tag.
+commit_file () {
+	_message=$1
+	_file=$2
+	_content=$3
+	stage_file "$_file" "$_content" &&
+	git commit -m "$_message" >/dev/null
+}
+
+# Create a fresh non-bare repository <name> in the trash directory and cd into
+# it.
+fresh_repo () {
+	_name=$1
+	cd "$SHARNESS_TRASH_DIRECTORY" &&
+	rm -rf "$_name" &&
+	git init -q "$_name" &&
+	cd "$_name"
+}
+
+# Create a repository <name>repo cloned from a bare <name>remote and cd into
+# it.
+clone_repo () {
+	_name=$1
+	cd "$SHARNESS_TRASH_DIRECTORY" &&
+	rm -rf "$_name"remote "$_name"repo &&
+	git init -q --bare "$_name"remote &&
+	git clone -q "$_name"remote "$_name"repo &&
+	cd "$_name"repo
+}
+
+# Create a repository tracked by a remote. The upstream is pushed after the
+# second commit, so the candidate search range covers "Add line three" and the
+# remote-tracking branch resolves for @{upstream}.
+#
+# Commits:
+#   Initial commit
+#   Add line two         <- origin/master
+#   Add line three
+setup_repo () {
+	(
+		# A subshell scopes errexit, so a failing git command stops the
+		# setup without && chains and without leaking state to the tests.
+		set -e
+		cd "$SHARNESS_TRASH_DIRECTORY"
+		rm -rf remote repo
+		git init -q --bare remote
+		git clone -q remote repo
+		cd repo
+		commit_file "Initial commit" file.txt 'line one\n'
+		commit_file "Add line two" file.txt 'line one\nline two\n'
+		git push -q -u origin master
+		commit_file "Add line three" file.txt 'line one\nline two\nline three\n'
+	) || return 1
+	# Tests leave the working directory inside the previous repo, so restore
+	# it for the next test body that cd's into the freshly created one.
+	cd "$SHARNESS_TRASH_DIRECTORY" || return 1
+}
+
+# Stage an extra line in file.txt
+stage_change () {
+	echo "$1" >>file.txt &&
+	git add file.txt
+}