options-permalink.php throws incorrect "You should update..." notice on multisite

[boonebgorges]

Setup:

1. Subdirectory multisite installation on Apache with mod_rewrite
2. /.htaccess/ and / should *not* be writable by the webserver
3. On a secondary site (NOT blog 1), Dashboard > Options > Permalinks, ensure that you have pretty permalinks enabled (not index.php, not default)
4. Click Save Changes without making any modifications
5. You should see a green notice "You should update your .htaccess file now"

Prior to #37380, saving existing settings would result in a success notice. However, #37380 just fixed a bug in the implementation of [37747], so I think the bug comes from #35936.

Prior to [37747], the "You should update..." family of notices was never shown when is_multisite(). (Multisite routing is taken care of by WordPress, not by the webserver; WP's generated RewriteBase etc are incorrect in multisite.) I think that the fix here is to reintroduce the is_multisite() check.

[boonebgorges]

See 39547.diff​. Ping @swissspidy for another set of eyes.

[SergeyBiryukov]

Previously: [13706] / #12604.

[SergeyBiryukov]

Found a related bug while looking into this: #50834.

[SergeyBiryukov]

In 48712:

Permalinks: Simplify the logic for displaying "You should update your .htaccess file now" message.

See #39547, #50834.

[SergeyBiryukov]

In 48713:

Permalinks: Don't show "You should update your .htaccess file now" message when saving permalink settings on Multisite.

Multisite routing is taken care of by WordPress, not by the web server.

Follow-up to [13706], [37747], [38066].

Props boonebgorges.
Fixes #39547.