Autocomplete on tab

[yoannfleurydev]

Is your feature request related to a problem? Please describe.

I'm frustrated when I'm using gitweb as it does not automatically autocomplete the option on TAB press.

Describe the solution you'd like

It could be awesome to have gitweb automatically autocomplete its options on TAB press.

Ressources:

• https://github.com/clap-rs/clap/blob/8749b0927aa7e56af330d275809b1a7edf9993d4/src/app/mod.rs#L1301
• https://github.com/TeXitoi/structopt/blob/master/examples/gen_completions.rs
• https://docs.rs/structopt/0.2.18/structopt/index.html

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