diff-closures: remove gratuitous copy

This was done originally because std::smatch does not accept `const char
*` as iterators. However, this was because we should have been using
std::cmatch instead.

(cherry picked from commit https://git.lix.systems/lix-project/lix/commit/12a5838d11f790d36e2ac626e8916a2c19bcb80b)

src/nix/diff-closures.cc

@@ -25,24 +25,17 @@ GroupedPaths getClosureInfo(ref<Store> store, const StorePath & toplevel)
 
     GroupedPaths groupedPaths;
 
-    for (auto & path : closure) {
+    for (auto const & path : closure) {
         /* Strip the output name. Unfortunately this is ambiguous (we
            can't distinguish between output names like "bin" and
            version suffixes like "unstable"). */
         static std::regex regex("(.*)-([a-z]+|lib32|lib64)");
-        std::smatch match;
+        std::cmatch match;
         std::string name{path.name()};
-        // Used to keep name alive through being potentially overwritten below
-        // (to not invalidate the references from the regex result)
-        //
-        // n.b. cannot be just path.name().{begin,end}() since that returns const
-        // char *, which does not, for some reason, convert as required on
-        // libstdc++. Seems like a libstdc++ bug or standard bug to me... we
-        // can afford the allocation in any case.
-        const std::string origName{path.name()};
+        std::string_view const origName = path.name();
         std::string outputName;
 
-        if (std::regex_match(origName, match, regex)) {
+        if (std::regex_match(origName.begin(), origName.end(), match, regex)) {
             name = match[1];
             outputName = match[2];
         }