Merge branch 'lexer_error_workaround' into 'master'

workaround for buggy lexers

It seems that some lexers can be buggy when used together with git blame.
This results in a 500 error when a user tries to open the blame page in some cases.
The workaround is pretty simple. Not only fall back to plaintext when the file content can't be guessed, but also when the lexer fails.

This closes #1521 and #1646

/cc @DouweM @JobV It would be awesome to have this in a patch release for 7.11 or even in the release. But that's probably to late.

See merge request !683

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.11.0 (unreleased)
+  - Fall back to Plaintext when Syntaxhighlighting doesn't work. Fixes some buggy lexers (Hannes Rosenögger)
   - Fix broken view when viewing history of a file that includes a path that used to be another file (Stan Hu)
   - Don't show duplicate deploy keys
   - Fix commit time being displayed in the wrong timezone in some cases (Hannes Rosenögger)

app/helpers/blob_helper.rb

@@ -9,11 +9,13 @@ module BlobHelper
 
     begin
       lexer = Rugments::Lexer.guess(filename: blob_name, source: blob_content)
-    rescue Rugments::Lexer::AmbiguousGuess
+      result = formatter.format(lexer.lex(blob_content)).html_safe
+    rescue
       lexer = Rugments::Lexers::PlainText
+      result = formatter.format(lexer.lex(blob_content)).html_safe
     end
 
-    formatter.format(lexer.lex(blob_content)).html_safe
+    result
   end
 
   def no_highlight_files