completion: Don't list sub-refs unless fully matching

Only complete to subrefs is fully matching real part.  For example,
only match org.foo.Bar.Sources for "org.foo.Bar", "org.foo.Bar." or
"org.foo.Bar.S", but not for "org.foo" or other shorter prefixes.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f