Fix implementation of real_path to handle non-existing directories in /

Fixes #1937

src/file.cc

@@ -67,6 +67,7 @@ std::pair<StringView, StringView> split_path(StringView path)
 
 String real_path(StringView filename)
 {
+    kak_assert(not filename.empty());
     char buffer[PATH_MAX+1];
 
     StringView existing = filename;
@@ -74,22 +75,21 @@ String real_path(StringView filename)
 
     while (true)
     {
-        char* res = realpath(existing.zstr(), buffer);
+        if (char* res = realpath(existing.zstr(), buffer))
-        if (res)
         {
             if (non_existing.empty())
                 return res;
-            return format("{}/{}", res, non_existing);
+            return format("{}{}", res, non_existing);
         }
 
-        auto it = find(existing.rbegin(), existing.rend(), '/');
+        auto it = find(existing.rbegin() + 1, existing.rend(), '/');
         if (it == existing.rend())
         {
             char cwd[1024];
             return format("{}/{}", getcwd(cwd, 1024), filename);
         }
 
-        existing = StringView{existing.begin(), it.base()-1};
+        existing = StringView{existing.begin(), it.base()};
         non_existing = StringView{it.base(), filename.end()};
     }
 }

test/regression/1937-opening-missing-file-by-full-path-fails/cmd

@@ -0,0 +1 @@
+"aP

test/regression/1937-opening-missing-file-by-full-path-fails/in

@@ -0,0 +1 @@
+

test/regression/1937-opening-missing-file-by-full-path-fails/out

@@ -0,0 +1 @@
+/kakoune-inexisting-directory/missing-file

test/regression/1937-opening-missing-file-by-full-path-fails/rc

@@ -0,0 +1,4 @@
+eval -draft %{
+    edit /kakoune-inexisting-directory/missing-file
+    reg a %val{buffile}
+}