Make fixGitURL reject file:/foo/bar/ URL

This is a breaking change, but I think a good one. If we had allowed it,
Git/SCP and the rest of our code would have disagreed on what this URL
meant.

src/libutil-tests/url.cc

@@ -74,19 +74,6 @@ INSTANTIATE_TEST_SUITE_P(
                     .path = {"", "home", "me", "repo"},
                 },
         },
-        // Already file: scheme
-        // NOTE: Git/SCP treat this as a `<hostname>:<path>`, so we are
-        // failing to "fix up" this case.
-        FixGitURLParam{
-            .input = "file:/var/repos/x",
-            .expected = "file:/var/repos/x",
-            .parsed =
-                ParsedURL{
-                    .scheme = "file",
-                    .authority = std::nullopt,
-                    .path = {"", "var", "repos", "x"},
-                },
-        },
         // IPV6 test case
         FixGitURLParam{
             .input = "user@[2001:db8:1::2]:/home/file",
@@ -136,6 +123,17 @@ TEST(FixGitURLTestSuite, properlyRejectFileURLWithAuthority)
             testing::HasSubstrIgnoreANSIMatcher("file:// URL 'file://var/repos/x' has unexpected authority 'var'")));
 }
 
+TEST(FixGitURLTestSuite, ambiguousScpLikeOrFileURL)
+{
+    /* Git/SCP treat this as a `<hostname>:<path>`, but under IETF RFC
+       8089 it is a valid (if sloppy) file URL. Rather than decide who
+       is right, we just make it an error. */
+    EXPECT_THAT(
+        []() { fixGitURL("file:/var/repos/x"); },
+        ::testing::ThrowsMessage<BadURL>(testing::HasSubstrIgnoreANSIMatcher(
+            "URL 'file:/var/repos/x' would parse as SCP authority = 'file', path = '/var/repos/x' but this is also a valid `file:..` URL, and so we choose to disallow it")));
+}
+
 TEST(FixGitURLTestSuite, scpLikePathLeadingSlashParsesPoorly)
 {
     // SCP-like URL (no user)

src/libutil/url.cc

@@ -413,8 +413,15 @@ ParsedURL fixGitURL(const std::string & url)
     std::regex scpRegex("([^/]*)@(.*):(.*)");
     if (!hasPrefix(url, "/") && std::regex_match(url, scpRegex))
         return parseURL(std::regex_replace(url, scpRegex, "ssh://$1@$2/$3"));
-    if (hasPrefix(url, "file:"))
+    std::string_view path = url;
-        return parseURL(url);
+    if (splitPrefix(path, "file:")) {
+        if (hasPrefix(url, "file://"))
+            return parseURL(url);
+        throw BadURL(
+            "URL '%s' would parse as SCP authority = 'file', path = '%s' but this is also a valid `file:..` URL, and so we choose to disallow it",
+            url,
+            path);
+    }
     if (url.find("://") == std::string::npos) {
         return ParsedURL{
             .scheme = "file",