chore: ignore detached HEAD (#950)

<!-- For Coveo Employees only. Fill this section.

CDX-1184

-->

## Proposed changes

When doing rebases with rewrite actions, commit-msg is a pain because he
doesn't find HEAD with the current command and crash, breaking the
rebase 

To solves this, I swapped the command with a similar one found on the
Internet (SWE101)

It returns `HEAD` when in a detached HEAD
It returns `refs/heads/branch-name` otherwise.

hooks/commit-msg.js

@@ -12,11 +12,15 @@ const urlBaseRegex = /https:\/\/coveord\.atlassian\.net\/browse/;
 const projectAcronym = 'CDX';
 
 let issueNumber;
-const branchName = childProcess
-  .execSync('git symbolic-ref --short HEAD', {encoding: 'utf8'})
+let branchName = childProcess
+  .execSync('git rev-parse --symbolic-full-name HEAD', {encoding: 'utf8'})
   .trim();
 const issueRegex = new RegExp(projectAcronym + '-[\\d]+', 'i');
-
+if (branchName === 'HEAD') {
+  process.exit();
+} else {
+  branchName = branchName.split('refs/heads/')[1];
+}
 const match = branchName.match(issueRegex);
 if (match) {
   issueNumber = match[0].toUpperCase();