Revert "CMake: use git rev-parse to get GIT_COMMIT_HASH"

This reverts commit f1e47e6.
Since unfortunately it breaks the addition of the git commit hash
to the version string when building from the VisualStudio IDE.
I presume this happens because `git` is not of the "path" of the
build environment. The version string ends up "3.0~" only.

cmake/GetGitCommitHash.cmake

@@ -1,11 +1,31 @@
 function(get_git_commit_hash)
-    execute_process(COMMAND git rev-parse --git-dir OUTPUT_VARIABLE GIT_DIR OUTPUT_STRIP_TRAILING_WHITESPACE)
-    execute_process(COMMAND git rev-parse HEAD OUTPUT_VARIABLE HEAD_REF OUTPUT_STRIP_TRAILING_WHITESPACE)
+    get_filename_component(GIT_DESCRIBE_CMAKE_DIR ${CMAKE_CURRENT_LIST_FILE} PATH)
+    get_filename_component(GIT_ROOT ${GIT_DESCRIBE_CMAKE_DIR} PATH)
+    set(GIT_DIR "${GIT_ROOT}/.git")
 
     # Add a CMake configure dependency to the currently checked out revision.
     set(GIT_DEPENDS ${GIT_DIR}/HEAD)
+    file(READ ${GIT_DIR}/HEAD HEAD_REF)
+    if(HEAD_REF MATCHES "ref: (.+)\n")
+        set(HEAD_REF ${CMAKE_MATCH_1})
+        if(EXISTS "${GIT_DIR}/${HEAD_REF}")
+            list(APPEND GIT_DEPENDS ${GIT_DIR}/${HEAD_REF})
+            file(READ ${GIT_DIR}/${HEAD_REF} HEAD_REF)
+        elseif(EXISTS "${GIT_DIR}/packed-refs")
+            list(APPEND GIT_DEPENDS ${GIT_DIR}/packed-refs)
+            file(READ "${GIT_DIR}/packed-refs" PACKED_REFS)
+            if(${PACKED_REFS} MATCHES "([0-9a-z]*) ${HEAD_REF}")
+                set(HEAD_REF ${CMAKE_MATCH_1})
+            else()
+                set(HEAD_REF "")
+            endif()
+        else()
+            set(HEAD_REF "")
+        endif()
+    endif()
     set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS ${GIT_DEPENDS})
 
+    string(STRIP ${HEAD_REF} HEAD_REF)
     if(HEAD_REF STREQUAL "")
         message(WARNING "Cannot determine git HEAD")
     else()