Database backup fails if there are spaces in the path to web root

[steverowling]

Description

When following the steps to upgrade a C2 site to C3 and make the directory structure like a C3 installation (https://github.com/craftcms/docs/blob/master/en/upgrade.md#if-you-want-your-directory-structure-to-resemble-a-new-craft-3-project), I successfully got to the end and had a "Finish up" command to update my database.

When clicking the button to finish up, the database backup failed, with the following entry in the web.log:

2018-01-05 11:08:36 [::1][-][-][error][craft\controllers\UpdaterController::actionBackup] Error backing up the database: The shell command "mysqldump --defaults-extra-file=/Users/steve/Dropbox/Work/Springworks/2018 website redesign/springworks/storage/runtime/temp/my.cnf --add-drop-table --comments --create-options --dump-date --no-autocommit --routines --set-charset --triggers --single-transaction --no-data --result-file=/Users/steve/Dropbox/Work/Springworks/2018 website redesign/springworks/storage/backups/springworks_180105_110836_08vsfeoyeg_v3.0.0-rc4.sql springworks-craft3 && mysqldump --defaults-extra-file=/Users/steve/Dropbox/Work/Springworks/2018 website redesign/springworks/storage/runtime/temp/my.cnf --add-drop-table --comments --create-options --dump-date --no-autocommit --routines --set-charset --triggers --no-create-info --ignore-table=springworks-craft3.craft_assetindexdata --ignore-table=springworks-craft3.craft_assettransformindex --ignore-table=springworks-craft3.craft_cache --ignore-table=springworks-craft3.craft_sessions --ignore-table=springworks-craft3.craft_templatecaches --ignore-table=springworks-craft3.craft_templatecachecriteria --ignore-table=springworks-craft3.craft_templatecacheelements springworks-craft3 >> /Users/steve/Dropbox/Work/Springworks/2018 website redesign/springworks/storage/backups/springworks_180105_110836_08vsfeoyeg_v3.0.0-rc4.sql" failed with exit code 1: Could not open required defaults file: /Users/steve/Dropbox/Work/Springworks/2018
Fatal error in defaults handling. Program aborted

I then changed the path for the project from /Users/steve/Dropbox/Work/Springworks/2018 website redesign/springworks to /Users/steve/Dropbox/Work/Springworks/2018-website-redesign/springworks, reset the document root in MAMP Pro, reloaded the /admin page and tried the "Finish up" again, which then succeeded.

Steps to reproduce

1. Create a new project with a space somewhere in the path name.
2. Attempt to update a Craft installation that requires a DB backup and click on the "Finish up" button.

Additional info

• Craft version: Craft Client 3.0.0-RC4
• PHP version: 7.1.8 (via MAMP Pro on Mac)
• Database driver & version: MySQL 5.6.35 (via MAMP Pro on Mac)
• Plugins & versions: N/A