libutil/url: fix git+file:./ parse error

Previously, the "file:./" prefix was not correctly recognized in
fixGitURL; instead, it was mistaken as a file path, which resulted in a
parsed url of the form "file://file:./".

This commit fixes the issue by properly detecting the "file:" prefix.
Note, however, that unlike "file://", the "file:./" URI is _not_
standardized, but has been widely used to referred to relative file
paths. In particular, the "git+file:./" did work for nix<=2.18, and was
broken since nix 2.19.0.

Finally, this commit fixes the issue completely for the 2.19 series, but
is still inadequate for the 2.20 series due to new behaviors from the
switch to libgit2. However, it does improve the correctness of parsing
even though it is not yet a complete solution.

(cherry picked from commit 8594f3cd5a)

src/libutil/url.cc

@@ -171,16 +171,16 @@ std::string fixGitURL(const std::string & url)
     std::regex scpRegex("([^/]*)@(.*):(.*)");
     if (!hasPrefix(url, "/") && std::regex_match(url, scpRegex))
         return std::regex_replace(url, scpRegex, "ssh://$1@$2/$3");
-    else {
-        if (url.find("://") == std::string::npos) {
-            return (ParsedURL {
-                .scheme = "file",
-                .authority = "",
-                .path = url
-            }).to_string();
-        } else
-            return url;
+    if (hasPrefix(url, "file:"))
+        return url;
+    if (url.find("://") == std::string::npos) {
+        return (ParsedURL {
+            .scheme = "file",
+            .authority = "",
+            .path = url
+        }).to_string();
     }
+    return url;
 }
 
 // https://www.rfc-editor.org/rfc/rfc3986#section-3.1