templates: re-add template listing support

We used to have --style nosuch to list templates, but --style is now
merged with --template/-T where random strings are acceptable
templates. So we reserve 'list' to allow listing templates.

mercurial/cmdutil.py

@@ -1126,6 +1126,10 def gettemplate(ui, tmpl, style):
             tmpl = templater.parsestring(t, quoted=False)
         return tmpl, None
 
+    if tmpl == 'list':
+        ui.write(_("available styles: %s\n") % templater.stylelist())
+        raise util.Abort(_("specify a template"))
+
     # perhaps it's a path to a map or a template
     if ('/' in tmpl or '\\' in tmpl) and os.path.isfile(tmpl):
         # is it a mapfile for a style?

mercurial/commands.py

@@ -4129,6 +4129,10 def log(ui, repo, *pats, **opts):
 
           hg log -k bug --template "{rev}\\n"
 
+      - list available log templates::
+
+          hg log -T list
+
       - check if a given changeset is included is a tagged release::
 
           hg log -r "a21ccf and ancestor(1.9)"

tests/test-command-template.t

@@ -485,6 +485,11 Error if no style:
   (available styles: bisect, changelog, compact, default, phases, xml)
   [255]
 
+  $ hg log -T list
+  available styles: bisect, changelog, compact, default, phases, xml
+  abort: specify a template
+  [255]
+
 Error if style missing key:
 
   $ echo 'q = q' > t