help: fix summary for repeated/binary commands

[unrevre]

fix fcf539e: 'commands' command is repeated; commands which call binary
executables (e.g. pyenv-realpath.dylib) are also grabbed, which cause
help message parsing to fail. depending on the system locale, sed may
also emit an error.

remove extra 'commands' command; restrict help message parsing to files
of mime-type 'text/*t'.

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[ericvw]

This closes #1454.

[zed]

#1461 fixes the duplicate commands summary while keeping it at the top of the help output.

[native-api]

Superceded by #1525 , #1714