git p4: RCS expansion should not span newlines

This bug was introduced in cb585a9 (git-p4: keyword
flattening fixes, 2011-10-16).  The newline character
is indeed special, and $File$ expansions should not try
to match across multiple lines.

Based-on-patch-by: Chris Goard <cgoard@gmail.com>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Jeff King <peff@peff.net>

git-p4.py

@@ -227,7 +227,7 @@ def p4_keywords_regexp_for_type(base, type_mods):
         pattern = r"""
             \$              # Starts with a dollar, followed by...
             (%s)            # one of the keywords, followed by...
-            (:[^$]+)?       # possibly an old expansion, followed by...
+            (:[^$\n]+)?     # possibly an old expansion, followed by...
             \$              # another dollar
             """ % kwords
         return pattern