config: specify the extension when forcing a config option

I noticed that `hg config server --debug` said 'none: server.bundle1=False'.
8 jobs for topic/default/small_fixes in 6 minutes and 2 seconds
Status Job ID Name Coverage
  Test
passed #103081
checks-py2

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passed #103082
checks-py3

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passed #103087
doc

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manual #103080
allowed to fail manual
pytype
passed #103083
tests-py2-cext

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passed #103084
tests-py2-pure

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passed #103085
tests-py3-cext

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passed #103086
tests-py3-pure

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