Enable $Id$ expansion for files with the $Revision$ keyword

Git supports a limited $Id$ keyword expansion. This $Id$ tag is similar to
$Revision$ in SVN. We enable the $Id$ expansion only for files that use
$Revision$.

.gitattributes

@@ -0,0 +1,23 @@
+ext/mysqlnd/mysqlnd.h           ident
+ext/simplexml/simplexml.c       ident
+ext/iconv/php_iconv.h           ident
+ext/posix/posix.c               ident
+ext/recode/recode.c             ident
+ext/skeleton/create_stubs       ident
+ext/phar/phar/pharcommand.inc   ident
+ext/phar/phar.c                 ident
+ext/sysvmsg/sysvmsg.c           ident
+ext/enchant/enchant.c           ident
+ext/reflection/php_reflection.c ident
+ext/oci8/oci8.c                 ident
+ext/dba/libinifile/inifile.c    ident
+ext/dba/libflatfile/flatfile.c  ident
+ext/dba/libcdb/cdb_make.c       ident
+ext/dba/libcdb/cdb.c            ident
+ext/filter/filter.c             ident
+README.input_filter             ident
+run-tests.php                   ident
+sapi/nsapi/nsapi.c              ident
+sapi/continuity/capi.c          ident
+Zend/RFCs/002.txt               ident
+Zend/RFCs/003.txt               ident