ExternalProject: Always do a git fetch for a remote ref.

Remote git refs always require a git fetch, because the remote may move around
where the ref points.

Modules/ExternalProject.cmake

@@ -418,6 +418,19 @@ if(error_code)
   message(FATAL_ERROR \"Failed to get the hash for HEAD\")
 endif()
 
+execute_process(
+  COMMAND \"${git_EXECUTABLE}\" show-ref ${git_tag}
+  WORKING_DIRECTORY \"${work_dir}\"
+  OUTPUT_VARIABLE show_ref_output
+  )
+# If a remote ref is asked for, which can possibly move around,
+# we must always do a fetch and checkout.
+if(\"\${show_ref_output}\" MATCHES \"remotes\")
+  set(is_remote_ref 1)
+else()
+  set(is_remote_ref 0)
+endif()
+
 # This will fail if the tag does not exist (it probably has not been fetched
 # yet).
 execute_process(
@@ -428,7 +441,7 @@ execute_process(
   )
 
 # Is the hash checkout out that we want?
-if(error_code OR NOT (\"\${tag_sha}\" STREQUAL \"\${head_sha}\"))
+if(error_code OR is_remote_ref OR NOT (\"\${tag_sha}\" STREQUAL \"\${head_sha}\"))
   execute_process(
     COMMAND \"${git_EXECUTABLE}\" fetch
     WORKING_DIRECTORY \"${work_dir}\"

Tests/ExternalProjectUpdate/ExternalProjectUpdateTest.cmake

@@ -29,12 +29,12 @@ macro(check_a_tag desired_tag resulting_sha)
     WORKING_DIRECTORY ${ExternalProjectUpdate_BINARY_DIR}/CMakeExternals/Source/TutorialStep1-GIT
     RESULT_VARIABLE error_code
     OUTPUT_VARIABLE tag_sha
+    OUTPUT_STRIP_TRAILING_WHITESPACE
     )
   if(error_code)
     message(FATAL_ERROR "Could not check the sha.")
   endif()
 
-  string(STRIP "${tag_sha}" tag_sha)
   if(NOT (${tag_sha} STREQUAL ${resulting_sha}))
     message(FATAL_ERROR "UPDATE_COMMAND produced
   ${tag_sha}
@@ -55,4 +55,6 @@ if(GIT_EXECUTABLE)
   check_a_tag(tag2          5842b503ba4113976d9bb28d57b5aee1ad2736b7)
   check_a_tag(d19707303     d1970730310fe8bc07e73f15dc570071f9f9654a)
   check_a_tag(origin/master 5842b503ba4113976d9bb28d57b5aee1ad2736b7)
+  # This is a remote symbolic ref, so it will always trigger a 'git fetch'
+  check_a_tag(origin/master 5842b503ba4113976d9bb28d57b5aee1ad2736b7)
 endif()