filemerge: introduce :x as short version of internal:x merge tool name

"internal:" is so much typing - it is frequently specified on the command line
and gives options that are longer than any other I have seen.

mercurial/filemerge.py

@@ -28,6 +28,7 b' def internaltool(name, trymerge, onfailu'
         fullname = 'internal:' + name
         func.__doc__ = "``%s``\n" % fullname + func.__doc__.strip()
         internals[fullname] = func
+        internals[':' + name] = func
         func.trymerge = trymerge
         func.onfailure = onfailure
         return func

tests/test-merge-tools.t

@@ -425,7 +425,7 b' ui.merge specifies internal:fail:'
   # hg stat
   M f
 
-ui.merge specifies internal:local:
+ui.merge specifies :local (without internal prefix):
 
   $ beforemerge
   [merge-tools]
@@ -433,7 +433,7 b' ui.merge specifies internal:local:'
   true.priority=1
   true.executable=cat
   # hg update -C 1
-  $ hg merge -r 2 --config ui.merge=internal:local
+  $ hg merge -r 2 --config ui.merge=:local
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ aftermerge