[core] Support paths starting with two slashes

Fixes ROOT-5430

Author: dpiparo

core/base/src/TUrl.cxx

@@ -131,13 +131,20 @@ void TUrl::SetUrl(const char *url, Bool_t defaultIsFile)
    fHostFQ     = "";
 
    // if url starts with a / consider it as a file url
-   if (url[0] == '/')
+   if (url[0] == '/') {
       defaultIsFile = kTRUE;
+      // ROOT-5430: if url starts with two slashes but
+      // not three slashes, just remove the first of them
+      if (strlen(url) > 2 && url[1] == '/' && url[2] != '/') {
+         url = &url[1];
+      }
+   }
 
    // Find protocol
    char *s, sav;
 
    TString surl = url;
+
    char *u, *u0 = Strip(defaultIsFile && surl.EndsWith(":/") ? TString(surl(0,surl.Length()-2)).Data() : url);
 tryfile:
    u = u0;

core/base/test/TUrlTest.cxx

@@ -12,4 +12,10 @@ TEST(TUrl, FilePath)
    //https://its.cern.ch/jira/browse/ROOT-5820
    TUrl u2("/tmp/a.root:/", kTRUE); // TFile.GetPath() returns a trailing :/
    EXPECT_TRUE(u2.IsValid());
+
+   // ROOT-5430
+   const char * ref_5430 = "file:///tmp/t.root";
+   EXPECT_STREQ(TUrl("/tmp/t.root").GetUrl(), ref_5430);
+   EXPECT_STREQ(TUrl("//tmp/t.root").GetUrl(), ref_5430);
+   EXPECT_STREQ(TUrl("///tmp/t.root").GetUrl(), ref_5430);
 }