clippy auto-fixes

[jgarzik]

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[Copilot]

Pull request overview

This PR applies automated clippy fixes across 14 Rust files to improve code quality and idiomatic usage. The changes are all safe refactorings that maintain the same behavior while making the code cleaner and more efficient.

• Simplified control flow structures (collapsing nested if statements, converting match to if-let)
• Removed redundant operations (unnecessary derefs, casts, allocations, and closures)
• Applied Rust idioms (lifetime elision, field init shorthand, String::repeat)

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
users/talk.rs	Removed redundant deref operator on mutable reference
users/pwd.rs	Collapsed nested if-else into single else-if statement
users/newgrp.rs	Applied multiple fixes: or_else→map_err, unwrap_or_else→unwrap_or, removed unnecessary reference parameters
tree/touch.rs	Converted match with empty None arm to cleaner if-let pattern
tree/common/copy.rs	Collapsed nested if-else into single else-if statement
tree/chown.rs	Removed unnecessary return statement in final expression
text/comm.rs	Collapsed nested if conditions into single compound expressions
process/kill.rs	Removed redundant cast as sig_no is already i32
mailx/main.rs	Simplified closure to direct function reference
mailx/mailbox.rs	Removed unnecessary to_string() call when &str is sufficient
ftw/src/dir.rs	Applied lifetime elision replacing explicit 'a with '_
fs/df.rs	Used field init shorthand for fields parameter
editors/vi/editor.rs	Replaced iterator::repeat pattern with more efficient String::repeat
datetime/cal.rs	Removed redundant cast as month() already returns u32
calc/expr.rs	Flattened nested else-if chain for better readability

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