Fix path to .gitmessage file in commit.template config

The `commit.template` config value is interpreted as a path relative to
the repository's root directory. Before, this config value stored the
path `.git/.gitmessage` in all cases. This is the correct path to the
`.gitmessage` file for normal repositories. However, if the repository
is a submodule of another repository, the `.gitmessage` file will be
stored in the `modules` subdirectory of the parent repository's `.git`
directory.

The solution here is to correctly compute a path that, when resolved
from the proejct's root directory, will resolve to the `.gitmessage`
file in all cases.

src/git-commands.js

@@ -120,6 +120,14 @@ function insideWorkTree() {
   return silentRun('git rev-parse --is-inside-work-tree').status === 0;
 }
 
+/**
+ * Computes the path to the top-level directory of the git repository.
+ * @returns {string} Path to the top-level directory of the git repository.
+ */
+function topLevelDirectory() {
+  return silentRun('git rev-parse --show-toplevel').stdout.trim();
+}
+
 module.exports = {
   version: gitVersion,
   config: {
@@ -133,5 +141,6 @@ module.exports = {
   revParse: {
     gitPath,
     insideWorkTree,
+    topLevelDirectory,
   },
 };

src/git-message/index.js

@@ -1,5 +1,6 @@
 const fs = require('fs');
 const os = require('os');
+const path = require('path');
 
 const { config, revParse } = require('../git-commands');
 
@@ -75,7 +76,7 @@ function commitTemplatePath() {
   return (
     process.env.GITMOB_MESSAGE_PATH ||
     config.get('commit.template') ||
-    '.git/.gitmessage'
+    path.relative(revParse.topLevelDirectory(), gitMessagePath())
   );
 }