Refactor merge/update git command calls (#23366)

Refactor merge/update git command calls (#23366)

Follow #22568

* Remove unnecessary ToTrustedCmdArgs calls
    * the FAQ in  #22678
* Quote: When using ToTrustedCmdArgs, the code will be very complex (see
the changes for examples). Then developers and reviewers can know that
something might be unreasonable.
* The `signArg` couldn't be empty, it's either `-S{keyID}` or
`--no-gpg-sign`.
* Use `signKeyID` instead, add comment "empty for no-sign, non-empty to
sign"
* 5-line code could be extracted to a common `NewGitCommandCommit()` to
handle the `signKeyID`, but I think it's not a must, current code is
clear enough.