fix conflicts between git-completions.nu and git-aliases.nu

[fdncred]

This PR fixes the conflicts between git-completions.nu and git-aliases.nu.

Prior to this PR you would see problems like this

❯ use custom-completions\git\git-completions.nu *
❯ use aliases\git\git-aliases.nu
Error: nu::parser::missing_flag_param

  × Missing flag argument.
    ╭─[D:\nu_scripts\aliases\git\git-aliases.nu:64:1]
 64 │ export alias gco = git checkout
 65 │ export alias gcor = git checkout --recurse-submodules
    ·                                  ──────────┬─────────
    ·                                            ╰── flag missing string argument
 66 │ export alias gcount = git shortlog --summary --numbered
    ╰────

This is because, in this example, in git-completions.nu, there is a custom command named git checkout that takes a --recurse-submodules: string parameter, which means it's expecting a string. Removing the : string part fixes the issue and allows the files to be sourced/used as expected.

This seems more like a hack than a fix. I'm not sure if this behavior is intended or not, but this PR fixes it anyway.

close #493