IntelliJ commit 50-character line length reminder

For newer IntelliJ versions (at least 2017 and above) read the (self-)answer of @gandreadis.

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In older IntelliJ version you can set Right margin for commit messages to 50 characters in Settings/Version Control.

Then the commit message input should look like this:

For anyone still interested in this: I've come across a new setting which does allow what I initially wanted, in a settings group called 'Commit Dialog'.

You can now have it warn you about a long subject line right in the dialog, without needing the visual aid of a delimiter!