gitaly: Fix migration of gitconfig with subsections

What does this MR do?

As we're migrating to the new gitaly['gitconfig'] format, we have implemented a migration that automatically converts from the old-style format in omnibus_gitconfig['system'] to the new format. The migration didn't take into account gitconfig with subsections of the form http."http://example.com".insteadOf though, so the migrated gitconfig is broken and causes errors.

Fix this bug by carefully trying to split up the old-style configuration into its different components.

Related issues

Prereq for !6447 (closed).

Related to #6800 (closed).

Related to gitaly#4230 (closed).

Part of &8069 (closed).

Related to #6889.

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
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• Equivalent MR/issue for the GitLab Chart opened