Enforce settings previously specified in a `.gitconfig`

### Is your feature request related to a problem? Please describe.
When I configure git (either system wide or project wide) I expected lazygit to respect all of my git config settings. However to my surprise, lazygit would not sign off commits unless explicitly set in it's OWN config.

### Describe the solution you'd like
If I have configured a setting in git, lazygit should respect it. I.e. if I have
```gitconfig
format.signOff = true
```
Then lazygit should automatically signoff all commits, without having to be explicitly configured to do so.

### Describe alternatives you've considered
I can work around this in Nix by just reading my own git config defined in nix. Though this is more of a bandaid than a real solution.

### Additional context
This feature request could extend to more than just `format.signOff` to ensure lazygit operates in parity with the users existing git config. As when I have configured git, it IS the expectation that my git config would apply everywhere I am using git... including lazygit.

Start by tracing how lazygit invokes commit operations and how it currently handles its own configuration, then compare that behavior with Git's existing configuration for format.signOff. Done means a Git setting such as format.signOff is respected by lazygit commits, with the broader set of settings to support clarified during implementation.

Written by the indexing model from the issue text.