Drop the git config cache when getting focus

This allows changing git config values while lazygit is running (e.g. in a
different terminal tab, or even in lazygit's ":" shell prompt), and have them
take effect immediately, while still getting some benefit from caching them
while lazygit is in the foreground.

pkg/commands/git_config/fake_git_config.go

@@ -27,3 +27,6 @@ func (self *FakeGitConfig) GetGeneral(args string) string {
 func (self *FakeGitConfig) GetBool(key string) bool {
 	return isTruthy(self.Get(key))
 }
+
+func (self *FakeGitConfig) DropCache() {
+}