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.

modules/git/command.go

@@ -179,7 +179,7 @@ func (c *Command) AddDashesAndList(list ...string) *Command {
 }
 
 // ToTrustedCmdArgs converts a list of strings (trusted as argument) to TrustedCmdArgs
-// In most cases, it shouldn't be used. Use AddXxx function instead
+// In most cases, it shouldn't be used. Use NewCommand().AddXxx() function instead
 func ToTrustedCmdArgs(args []string) TrustedCmdArgs {
 	ret := make(TrustedCmdArgs, len(args))
 	for i, arg := range args {