Guess if .r is R or REBOL

.r is the default extension used widely for both, R and REBOL. This
patch attempts to guess if .r is a REBOL file by looking for the
following very common REBOL code fragments:

- `REBOL`
- `: func [`
- `make object! [`
- `context [`

If any of those is found in a .r file, it is identified as a REBOL file.
Otherwise the language for a .r file is R.

Signed-off-by: Andreas Bolka <a@bolka.at>

lib/linguist/blob_helper.rb

@@ -259,6 +259,9 @@ module Linguist
       # If its a header file (.h) try to guess the language
       header_language ||
 
+        # If it's a .r file, try to guess the language
+        r_language ||
+
         # See if there is a Language for the extension
         pathname.language ||
 
@@ -288,6 +291,19 @@ module Linguist
       end
     end
 
+    # Internal: Guess language of .r files.
+    #
+    # Returns a Language.
+    def r_language
+      return unless extname == '.r'
+
+      if lines.grep(/(rebol|(:\s+func|make\s+object!|^\s*context)\s*\[)/i).any?
+        Language['Rebol']
+      else
+        Language['R']
+      end
+    end
+
     # Internal: Extract the script name from the shebang line
     #
     # Requires Blob#data