readHead(): Make sure we're returning the HEAD ref line

If we previously fetched by revision, the output of "git ls-remote"
won't start with the expected line like

  ref: refs/heads/master HEAD

but will be something like

  5c4410e3b9891c05ab40d723de78c6f0be45ad30        refs/heads/5c4410e3b9891c05ab40d723de78c6f0be45ad30

This then causes Nix to treat that revision as a refname, which then
leads to warnings like

  warning: could not update cached head '5c4410e3b9891c05ab40d723de78c6f0be45ad30' for 'file:///tmp/repo'

src/libfetchers/git.cc

@@ -69,7 +69,7 @@ std::optional<std::string> readHead(const Path & path)
 
     std::string_view line = output;
     line = line.substr(0, line.find("\n"));
-    if (const auto parseResult = git::parseLsRemoteLine(line)) {
+    if (const auto parseResult = git::parseLsRemoteLine(line); parseResult && parseResult->reference == "HEAD") {
         switch (parseResult->kind) {
             case git::LsRemoteRefLine::Kind::Symbolic:
                 debug("resolved HEAD ref '%s' for repo '%s'", parseResult->target, path);