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.
Drag & drop error message fixed
Add Clean cache
fix the test CanGetRelativeDateString failing on seconds round up
Add GitExtensionsDoc submodule
Corrected source location link
Updated submoduls
Merge pull request #1553 from fraga/addCacheClean

Add Clean cache
Stage submodule in commit dialog fixed for some situations
Submodule status added to diff window
We shouldn't use tab but white spaces to indent the code
White spaces replacing tab to indent code
Merge pull request #1559 from fraga/addCacheClean

White spaces replacing tab to indent code
Use read-only urls for submoduls.
Add TranslationApp to vs2010 sln
Rebase patch files decoding fixed for UTF8 strings

ReadAllLines replaced by ReadLines in several places
Merge pull request #1554 from fraga/fixCanGetRelativeDateTest

fix the test CanGetRelativeDateString failing on seconds round up