CMakeVersion: Preserve Git information during 'git archive'

Use `export-subst` to get information about the Git commit used to
produce an archive.

Source/.gitattributes

@@ -1,2 +1,4 @@
+CMakeVersion.cmake    export-subst
+
 # Do not format third-party sources.
 /kwsys/**                                  -format.clang-format-6.0

Source/CMakeVersion.cmake

@@ -39,6 +39,9 @@ if(COMMAND _git)
   # Get the commit checked out in this work tree.
   _git(log -n 1 HEAD "--pretty=format:%h %s" --)
   set(git_info "${_git_out}")
+else()
+  # Get the commit exported by 'git archive'.
+  set(git_info [==[$Format:%h %s$]==])
 endif()
 
 # Extract commit information if available.