add CMake funcion for determining current git rev

/GetGitHash.cmake has function get_git_hash that gives current
git rev hash or "NO_GIT_COMMIT_HASH_DEFINED" if we are
building out of git repo.
It's used in /renderdoc/CMakeLists.txt

GetGitHash.cmake

@@ -0,0 +1,13 @@
+function(get_git_hash _git_hash)
+  if(EXISTS "${CMAKE_SOURCE_DIR}/.git")
+    execute_process(
+      COMMAND git rev-parse HEAD
+      WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+      OUTPUT_VARIABLE GIT_HASH
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+    )
+  else()
+    set(GIT_HASH "NO_GIT_COMMIT_HASH_DEFINED")
+  endif()
+  set(${_git_hash} "${GIT_HASH}" PARENT_SCOPE)
+endfunction(get_git_hash)

renderdoc/CMakeLists.txt

@@ -1,3 +1,5 @@
+include("${CMAKE_SOURCE_DIR}/GetGitHash.cmake")
+
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 
 set(RDOC_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
@@ -8,8 +10,7 @@ set(RDOC_INCLUDES
 set(RDOC_LIBRARIES)
 
 # get git commit hash
-execute_process(COMMAND sh ${CMAKE_SOURCE_DIR}/calc_hash.sh
-    OUTPUT_VARIABLE GIT_COMMIT_HASH)
+get_git_hash(GIT_COMMIT_HASH)
 string(STRIP ${GIT_COMMIT_HASH} GIT_COMMIT_HASH)
 list(APPEND RDOC_DEFINITIONS
     PRIVATE -DGIT_COMMIT_HASH="${GIT_COMMIT_HASH}")