Add an integration test for a config with a negative refspec

[radsaq]

• PR Description

This is a sanity check to verify that lazygit has been built with a patched version of go-git that does not error on negative refspecs.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[radsaq]

I added a git version condition.

[stefanhaller]

Nice, looks good. I verified that the test fails when f03e1e4 is reverted. Merging.