Improve ref validity checking in fetchGit

The previous regex was too strict and did not match what git was allowing. It
could lead to `fetchGit` not accepting valid branch names, even though they
exist in a repository (for example, branch names containing `/`, which are
pretty standard, like `release/1.0` branches).

The new regex defines what a branch name should **NOT** contain. It takes the
definitions from `refs.c` in https://github.com/git/git and `git help
check-ref-format` pages.

This change also introduces a test for ref name validity checking, which
compares the result from Nix with the result of `git check-ref-format --branch`.

src/libfetchers/git.cc

@@ -418,7 +418,7 @@ struct GitInputScheme : InputScheme
 
         auto input = std::make_unique<GitInput>(parseURL(getStrAttr(attrs, "url")));
         if (auto ref = maybeGetStrAttr(attrs, "ref")) {
-            if (!std::regex_match(*ref, refRegex))
+            if (std::regex_search(*ref, badGitRefRegex))
                 throw BadURL("invalid Git branch/tag name '%s'", *ref);
             input->ref = *ref;
         }