/deploy <package>: do not assume that req.body.comment.body is a string

Apparently it is not, as I got this response for
git-for-windows/build-extra#455 (comment):

	TypeError: Cannot read properties of undefined (reading 'startsWith')

Let's just do the same as for the `/mention` command: use regular
expressions. If we use only greedy expressions, we should not run into
the same issues as caused the scary StackOverflow outage described in
https://stackstatus.tumblr.com/post/147710624694/outage-postmortem-july-20-2016.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

GitForWindowsHelper/slash-commands.js

@@ -103,8 +103,8 @@ module.exports = async (context, req) => {
             return `I edited the comment: ${commentURL}`
         }
 
-        const deployArg = command.startsWith('/deploy ') ? command.substring(8).trim() : undefined
-        if (deployArg || command === '/deploy') {
+        const deployMatch = command.match(/^\/deploy(\s+(\S+)\s*)?$/)
+        if (deployMatch) {
             if (owner !== 'git-for-windows'
              || !req.body.issue.pull_request
              || !['build-extra', 'MINGW-packages', 'MSYS2-packages'].includes(repo)) {
@@ -114,7 +114,7 @@ module.exports = async (context, req) => {
             await checkPermissions()
 
             const { guessComponentUpdateDetails, isMSYSPackage } = require('./component-updates')
-            const { package_name } = deployArg || guessComponentUpdateDetails(req.body.issue.title, req.body.issue.body)
+            const { package_name } = deployMatch[2] || guessComponentUpdateDetails(req.body.issue.title, req.body.issue.body)
 
             // The commit hash of the tip commit is sadly not part of the
             // "comment.created" webhook's payload. Therefore, we have to get it