GitCompetion.ps1

Register-ArgumentCompleter -Native -CommandName git -ScriptBlock {
    param([string]$commandName, [string]$wordToComplete, [int]$cursorPosition)
    
    function MatchOnly([string] $cmd, [string[]]$candidate){
        $ret = $candidate -like "$cmd*"

        if ($ret.Length -eq 0) {
            return [System.String]::Empty
        } elseif ($ret.Length -eq 1) {
            return [System.Management.Automation.CompletionResult]::new($ret[0] + " ")
        } else {
            $l = [System.Int32]::MaxValue
            foreach($r in $ret) {
                if ($l -gt $r.Length) {
                    $l = $r.Length
                }
            }

            [int] $ll = $cmd.Length

            :loop while($ll -lt $l) {
                foreach ($r in $ret) {
                    if ($r[$ll] -ne $ret[0][$ll]) {
                        break loop
                    }
                }
                $ll += 1
            }

            if ($ll -ne $cmd.Length) {
                return [System.Management.Automation.CompletionResult]::new($ret[0].Substring(0, $ll))
            }
        }
    }

    function CompleteGitCommand {
        param([string] $cmd)

        [string[]]$gitCommands = @(
            "clone", "init", "add", "mv", "reset",
            "rm", "bisect", "grep", "log", "show",
            "status", "branch", "checkout", "commit", "diff",
            "merge", "rebase", "tag", "fetch", "pull",
            "push", "clean", "send-email", "am", "shortlog",
            "apply", "gui", "range-diff", "archive", "config",
            "help", "show-branch", "describe", "reflog", "stage",
            "blame", "instaweb", "remote", "stash", "difftool",
            "lfs", "repack", "bundle", "replace", "submodule",
            "flow", "request-pull", "cherry", "format-patch", "mergetool",
            "whatchanged", "cherry-pick", "fsck", "revert", "worktree",
            "citool", "gc", "notes")
        return MatchOnly $cmd $gitCommands
    }

    function CompleteHelpCommand ([string] $cmd) {
        return CompleteGitCommand $cmd
    }

    function CompleteCommitCommand ([string] $cmd) {
        [string[]] $commitArgument = @(
            "--ahead-behind",         "--include",              "--quiet",
            "--all",                  "--interactive",          "--reedit-message=",
            "--amend",                "--long",                 "--reset-author",
            "--author=",              "--message=",             "--reuse-message=",
            "--branch",               "--short",                "--no-fixup",
            "--cleanup=",             "--no-post-rewrite",      "--signoff",
            "--date=",                "--no-verify",            "--squash=",
            "--dry-run",              "--null",                 "--status",
            "--edit",                 "--only",                 "--template=",
            "--file=",                "--patch",                "--untracked-files",
            "--fixup=",               "--porcelain",            "--verbose",
            "--gpg-sign",             "--post-rewrite",         "--verify",
            "--no-ahead-behind",      "--no-gpg-sign",          "--no-reedit-message",
            "--no-all",               "--no-include",           "--no-reset-author",
            "--no-amend",             "--no-interactive",       "--no-reuse-message",
            "--no-author",            "--no-long",              "--no-short",
            "--no-branch",            "--no-message",           "--no-signoff",
            "--no-cleanup",           "--no-null",              "--no-squash",
            "--no-date",              "--no-only",              "--no-status",
            "--no-dry-run",           "--no-patch",             "--no-template",
            "--no-edit",              "--no-porcelain",         "--no-untracked-files",
            "--no-file",              "--no-verbose",           "--no-quiet"
            )
        return MatchOnly $cmd $commitArgument
    }

    $all = $wordToComplete.Substring(0, $cursorPosition)
    $mid = $all.TrimStart()
    $mid = $mid.Substring(3, $mid.Length - 3 - $commandName.Length)
    $tokens = $all.Split(" ", 3)

    if ([System.String]::IsNullOrWhiteSpace($mid)) {
        return CompleteGitCommand $commandName
    } elseif ($tokens.Length -ge 2) {
        $command = $tokens[1]
        switch ($command) {
            "help" { return CompleteHelpCommand $commandName }
            "commit" { return CompleteCommitCommand $commandName }
        }
    }
}