Revert "core syntax: strip the path from filename on syntax.get (#1168)" (#1322)

* Revert "core syntax: strip the path from filename on syntax.get (#1168)"

This reverts commit af6c4bc152.

The previous behavior was correct and allowed access to the full path for path-dependant syntaxes.

* Use `Doc.abs_filename` to obtain syntax when possible

This allows matching full paths in language syntaxes, but we lose the
possibility of matching the project root.

data/core/doc/init.lua

@@ -44,7 +44,12 @@ end
 
 function Doc:reset_syntax()
   local header = self:get_text(1, 1, self:position_offset(1, 1, 128))
-  local syn = syntax.get(self.filename or "", header)
+  local path = self.abs_filename
+  if not path and self.filename then
+    path = core.project_dir .. PATHSEP .. self.filename
+  end
+  if path then path = common.normalize_path(path) end
+  local syn = syntax.get(path or "", header)
   if self.syntax ~= syn then
     self.syntax = syn
     self.highlighter:soft_reset()