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/syntax.lua

@@ -44,7 +44,7 @@ local function find(string, field)
 end
 
 function syntax.get(filename, header)
-  return find(common.basename(filename), "files")
+  return find(filename, "files")
       or (header and find(header, "headers"))
       or plain_text_syntax
 end