cmdutil: strip "%m" pattern (first line of commit message) from both ends

This matches the behavior of the template keyword {desc}.

mercurial/cmdutil.py

@@ -898,7 +898,7 b' def makefilename(ctx, pat,'
         'R': lambda: '%d' % ctx.rev(),
         'h': lambda: short(ctx.node()),
         'm': lambda: re.sub('[^\w]', '_',
-                            ctx.description().rstrip().splitlines()[0]),
+                            ctx.description().strip().splitlines()[0]),
         'r': lambda: ('%d' % ctx.rev()).zfill(revwidth or 0),
         '%': lambda: '%',
         'b': lambda: os.path.basename(ctx.repo().root),

tests/test-export.t

@@ -184,7 +184,7 b' Checking if only alphanumeric characters'
   $ hg commit -m " !\"#$%&(,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]"'^'"_\`abcdefghijklmnopqrstuvwxyz{|}~"
   $ hg export -v -o %m.patch tip
   exporting patch:
-  ____________0123456789_______ABCDEFGHIJKLMNOPQRSTUVWXYZ______abcdefghijklmnopqrstuvwxyz____.patch
+  ___________0123456789_______ABCDEFGHIJKLMNOPQRSTUVWXYZ______abcdefghijklmnopqrstuvwxyz____.patch
 
 Invalid pattern in file name: