string: Teach TIMESTAMP to treat %% as %

This encoding is documented by `strptime`.

Help/command/string.rst

@@ -296,6 +296,7 @@ specifiers:
 
 ::
 
+   %%        A literal percent sign (%).
    %d        The day of the current month (01-31).
    %H        The hour on a 24-hour clock (00-23).
    %I        The hour on a 12-hour clock (01-12).

Help/release/dev/timestamp-percent.rst

+timestamp-percent
+-----------------
+
+* The :command:`string(TIMESTAMP)` command learned to treat ``%%``
+  as a way to encode plain ``%``.

Source/cmTimestamp.cxx

@@ -136,6 +136,7 @@ std::string cmTimestamp::AddTimestampComponent(char flag,
     case 'w':
     case 'y':
     case 'Y':
+    case '%':
       break;
     case 's': // Seconds since UNIX epoch (midnight 1-jan-1970)
     {

Tests/RunCMake/string/Timestamp-stderr.txt

-RESULT=2005-08-07 23:19:49 Sun Aug 05 day=219 wd=0 week=32 %%I=11
+RESULT=2005-08-07 23:19:49 Sun Aug 05 day=219 wd=0 week=32 %I=11 epoch=1123456789

Tests/RunCMake/string/Timestamp.cmake

 set(ENV{SOURCE_DATE_EPOCH} "1123456789")
-string(TIMESTAMP RESULT "%Y-%m-%d %H:%M:%S %a %b %y day=%j wd=%w week=%U %%I=%I" UTC)
+string(TIMESTAMP RESULT "%Y-%m-%d %H:%M:%S %a %b %y day=%j wd=%w week=%U %%I=%I epoch=%s" UTC)
 message("RESULT=${RESULT}")