hghave: check darcs version more strictly

test-convert-darcs.t suddenly started failing on my Debian sid machine. The
reason was Darcs was upgraded from 2.12.0 to 2.12.4 so the original pattern
got to match the last two digits. Fix the pattern to match 2.2+.

tests/hghave.py

@@ -150,7 +150,7 b' def has_cvsnt():'
 
 @check("darcs", "darcs client")
 def has_darcs():
-    return matchoutput('darcs --version', br'2\.[2-9]', True)
+    return matchoutput('darcs --version', br'\b2\.([2-9]|\d{2})', True)
 
 @check("mtn", "monotone client (>= 1.0)")
 def has_mtn():