Enable git completion via <Tab> in custom command

**Is your feature request related to a problem? Please describe.**

Lazygit makes it easy to navigate submodules, however, running a complex command in the submodule usually requires `cd /path/to/submodule` first. It would be very handy to have git-completion inside a custom command.

Currently, pressing `` inside a custom command causes the application to change focus to an empty rectangle. I would really like it to use git-completion to save me some typing.

**Describe the solution you'd like**

Pressing `` should finish typing the reference, if it is a partial string match to anything known by git. If there are multiple matches, it should show them to me with ``.

**Describe alternatives you've considered**

A shortcut to open the terminal/command line in the directory of the active Git repository.

**Additional context**


`commit=f91adf026bfbddc9505d6d84d3fabc80d49c7ca0, build date=2021-06-05T04:16:14Z, build source=binaryRelease, version=0.28.2, os=linux, arch=amd64`