python: fix re2.search(compiled, text)

The old code was accidentally recompiling the pattern
and losing the options, both inefficient and incorrect.
Thanks to @MajorTanya for the excellent diagnosis.

This bug was introduced in CL 60290 and then copied
over from the Abseil branch.

Fixes #558.

Change-Id: I072707003b444fb0934a310e14147b4d65b9da11
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63830
Reviewed-by: Alan Donovan <adonovan@google.com>

python/re2.py

@@ -64,7 +64,7 @@ def compile(pattern, options=None):
     if options:
       raise error('pattern is already compiled, so '
                   'options may not be specified')
-    pattern = pattern._pattern
+    return pattern
   options = options or Options()
   values = tuple(getattr(options, name) for name in Options.NAMES)
   return _Regexp._make(pattern, values)