Use git to choose file on choose local/remote/base file in FormResolveConflicts

 - It is important to use git to checkout files because old existing methods in GitModule.cs
   can checkout wrong file content. This is still a problem in method SaveBlobAs and
   should also be fixed.
 - Fixes part of issue #1456
Use git to save/open a file on save/open in FormResolveConflicts

 - It is important to use git to checkout files because old existing methods in GitModule.cs
   can checkout wrong file content. This is still a problem in method SaveBlobAs and
   should also be fixed.
- Now it is also shown an error if open or save file failed
- Fixes part of issue #1456
Only show save as and open with in context menu of conflicted files if files are present
Use a more git compatible way to convert lf to crlf in GitModule.SaveBlobAs

- The old version of GitModule.SaveBlobAs removes file information like BOM,
  the new version of GitModule.SaveBlobAs treats the file more like a binary file.
 - GitModule.ConvertCrLfToWorktree is a better way to do line ending conversion
   in GitExtensions. It converts line endings in the fashion of git.
- But it doesn't checkout files like git will do it. Because it doesn't parse .gitattributes
  file and has not the same lf to crlf algorithm.
- The best way to check out a file in SaveBlobAs would be to use git checkout <commit-id> <name of
   file>. But that would overwrite current file in working directory.
Merge branch 'pr/n1457_fernsehkind'