Fix fg/bg extraction with l.t. 2 colors

[gramosg]

Hi, this PR fixes a bug I found because I have a configuration like this:

[color "diff"]
        old = red strike

In short, it wasn't considered the case where 1) less than 2 colors are specified, with 2) at least one modifier different from bold or reverse.

The bug can be reproduced by setting git config color.diff.old "red strike" in test/test_helper/util.bash and running the test suite, but I'm not familiar enough with bats to add a regression test, as it seems to affect the configuration before the tests run.

I hope it is useful :)

[gramosg]

Update: just realized about the next branch; I've already edited the PR (was targeting master).

[scottchiefbaker]

Looks good! All the tests still pass, and this will make us more resilient in the future. Thanks!