Expose git_repository_set_config function

[tt]

On my local machine, I set diff.noPrefix to true in my global Git configuration.

This can interfere with software that uses git2. For instance, running cargo test in this repository causes the diff::tests::format_email_simple test to fail as it's expecting lines with the "a/" and "b/" prefixes:

git2-rs/src/diff.rs

Lines 1694 to 1695 in 62e83e2

	"--- a/file1.txt\n",
	"+++ b/file1.txt\n",

There's a couple of workarounds available:

1. I can prevent the global configuration from loading by using git2::opts::set_search_path and pointing it at a non-existent path. The downside is that it is itself a global option.

2. I can (probably) use git2::DiffOptions to undo my local environment. This is a bit verbose but a great solution in many cases to ensure predictability

I'm proposing exposing the git_repository_set_config function to allow setting configuration for a particular repository instance. This gives the user full control of which configuration to load.

[weihanglo]

Yeah, this is useful for complete config isolation, but for your specific (the diff.noPrefix issue), shouldn't repo.config()?.set_bool("diff.noPrefix", false) help with that? That would override the global setting without removing all other inherited config.

Before merging, could you clarify why that didn't work? If it is a separate bug we might want to fix it.

[tt]

It works but it's very specific to running the software on my machine.

Rather than relying on Git's defaults, I have to unset everything that may be different in a particular environment like diff.context, diff.mnemonicPrefix etc.

So, yes, it works but I would prefer the ability to not load any configuration.

[weihanglo]

Yeah that makes sense. I was just wondering if you missed those API or there were bugs. Thanks for the clarification!