require: provide a link to a wiki page in addition of suggesting upgrade

The wiki page is intended to describe several solution to the requirement issue.
Some of those solutions does not involve upgrading mercurial. That is very
useful for people that can't easily upgrade they Mercurial in some place.

mercurial/scmutil.py

@@ -735,7 +735,9 b' def readrequires(opener, supported):'
     if missings:
         raise error.RequirementError(
             _("unknown repository format: requires features '%s' (upgrade "
-              "Mercurial)") % "', '".join(missings))
+              "Mercurial)") % "', '".join(missings),
+            hint=_("see http://mercurial.selenic.com/wiki/MissingRequirement"
+                   " for details"))
     return requirements
 
 class filecachesubentry(object):

tests/test-commit.t

@@ -103,6 +103,7 b' Make sure we do not obscure unknown requ'
   $ echo fake >> .hg/requires
   $ hg commit -m bla
   abort: unknown repository format: requires features 'fake' (upgrade Mercurial)!
+  (see http://mercurial.selenic.com/wiki/MissingRequirement for details)
   [255]
 
   $ cd ..

tests/test-identify.t

@@ -114,11 +114,13 b' Make sure we do not obscure unknown requ'
   $ echo fake >> .hg/requires
   $ hg id
   abort: unknown repository format: requires features 'fake' (upgrade Mercurial)!
+  (see http://mercurial.selenic.com/wiki/MissingRequirement for details)
   [255]
 
   $ cd ..
 #if no-outer-repo
   $ hg id test
   abort: unknown repository format: requires features 'fake' (upgrade Mercurial)!
+  (see http://mercurial.selenic.com/wiki/MissingRequirement for details)
   [255]
 #endif

tests/test-largefiles.t

@@ -2260,6 +2260,7 b' enabling largefiles extension.'
   $TESTTMP/individualenabling/enabledlocally (glob)
   $ hg -R notenabledlocally root
   abort: unknown repository format: requires features 'largefiles' (upgrade Mercurial)!
+  (see http://mercurial.selenic.com/wiki/MissingRequirement for details)
   [255]
 
   $ hg init push-dst
@@ -2276,6 +2277,7 b' enabling largefiles extension.'
 
   $ hg clone enabledlocally clone-dst
   abort: unknown repository format: requires features 'largefiles' (upgrade Mercurial)!
+  (see http://mercurial.selenic.com/wiki/MissingRequirement for details)
   [255]
   $ test -d clone-dst
   [1]

tests/test-requires.t

@@ -10,10 +10,12 b''
   $ echo indoor-pool > .hg/requires
   $ hg tip
   abort: unknown repository format: requires features 'indoor-pool' (upgrade Mercurial)!
+  (see http://mercurial.selenic.com/wiki/MissingRequirement for details)
   [255]
   $ echo outdoor-pool >> .hg/requires
   $ hg tip
   abort: unknown repository format: requires features 'indoor-pool', 'outdoor-pool' (upgrade Mercurial)!
+  (see http://mercurial.selenic.com/wiki/MissingRequirement for details)
   [255]
   $ cd ..
 
@@ -61,6 +63,7 b' another repository of push/pull/clone on'
 
   $ hg clone supported clone-dst
   abort: unknown repository format: requires features 'featuresetup-test' (upgrade Mercurial)!
+  (see http://mercurial.selenic.com/wiki/MissingRequirement for details)
   [255]
   $ hg clone --pull supported clone-dst
   abort: required features are not supported in the destination: featuresetup-test