revsets: introduce revsingle helper

revsingle returns a context for the last revision of the supplied revspec

mercurial/cmdutil.py

@@ -111,6 +111,15 b' def loglimit(opts):'
         limit = None
     return limit
 
+def revsingle(repo, revspec, default=None):
+    if not revspec:
+        return repo[default]
+
+    l = revrange(repo, [revspec])
+    if len(l) < 1:
+        raise util.Abort("empty revision set")
+    return repo[l[-1]]
+
 def revpair(repo, revs):
     if not revs:
         return repo.dirstate.parents()[0], None

mercurial/commands.py

@@ -634,7 +634,7 b' def cat(ui, repo, file1, *pats, **opts):'
 
     Returns 0 on success.
     """
-    ctx = repo[opts.get('rev')]
+    ctx = cmdutil.revsingle(repo, opts.get('rev'))
     err = 1
     m = cmdutil.match(repo, (file1,) + pats, opts)
     for abs in ctx.walk(m):