tests: use raw string in test-impexp-branch.t

On first glance, the escaping of \s seems correct. However,
the shell eats the escape and we're left with '\s` in the
written file. Let's use a raw string so we don't have to
double escape.

Differential Revision: https://phab.mercurial-scm.org/D5973

tests/test-impexp-branch.t

@@ -6,7 +6,7 b''
   > import re
   > import sys
   > 
-  > head_re = re.compile('^#(?:(?:\\s+([A-Za-z][A-Za-z0-9_]*)(?:\\s.*)?)|(?:\\s*))$')
+  > head_re = re.compile(r'^#(?:(?:\\s+([A-Za-z][A-Za-z0-9_]*)(?:\\s.*)?)|(?:\\s*))$')
   > 
   > for line in sys.stdin:
   >     hmatch = head_re.match(line)