[CMake] Update GetSVN.cmake to handle Repo

[minseong.kim]

Summary

When repo is used with git, 'clang --version' option does not display the correct revision information (i.e. git hashes on TOP) as the following:

clang version 6.0.0 (should be) --> clang version 6.0.0 (.../clang.git "clang's git hash on top") (.../llvm.git "llvm's git hash on top")

This is because repo also creates .git/svn folder as git-svn does and this makes repo with git uses "git svn info" command, which is only for git-svn, to retrieve its revision information, making null for the info. To correctly distinguish between git-svn and repo with git, the folder hierarchy to specify for git-svn should be .git/svn/refs as the "git svn info" command depends on the revision data in .git/svn/refs. This patch in turn makes repo with git passes through to the third macro, get_source_info_git, in get_source_info function, resulting in correctly retrieving the revision information for repo with git using "git log ..." command.

This patch is tested with git, svn, git-svn, and repo with git.

[minseong.kim]

kindly ping~

[minseong.kim]

I have updated the description with a hope for it to be more descriptive.
Kindly ping~

[probinson]

The patch seems trivial enough but I do not have a good idea of the subtle differences in the various environments.
I have subscribed a couple of people who probably can tell whether this change makes sense.

[rnk]

lgtm

llvm/trunk/cmake/modules/GetSVN.cmake

if (git_result EQUAL 0)
       set(${repository} "${git_url}" PARENT_SCOPE)
     endif()
   endif()
 endmacro()
 
 function(get_source_info path revision repository)
   if (EXISTS "${path}/.svn")
     get_source_info_svn("${path}" revision repository)
-  elseif (EXISTS "${path}/.git/svn")
+  elseif (EXISTS "${path}/.git/svn/refs")
     get_source_info_git_svn("${path}" revision repository)
   elseif (EXISTS "${path}/.git")
     get_source_info_git("${path}" revision repository)
   endif()
 endfunction()
 
 function(append_info name path)
   get_source_info("${path}" revision repository)