templater: handle exception when applying map operator to non-iterable object

Before this, "{noniterable % template}" raised an exception. This tries to
provide a better indication for the common case, where a left-hand-side
expression is a keyword.

mercurial/templater.py

@@ -305,9 +305,17 b' def runmap(context, mapping, data):'
     func, data, ctmpl = data
     d = func(context, mapping, data)
     if util.safehasattr(d, 'itermaps'):
-        d = d.itermaps()
+        diter = d.itermaps()
+    else:
+        try:
+            diter = iter(d)
+        except TypeError:
+            if func is runsymbol:
+                raise error.ParseError(_("keyword '%s' is not iterable") % data)
+            else:
+                raise error.ParseError(_("%r is not iterable") % d)
 
-    for i in d:
+    for i in diter:
         lm = mapping.copy()
         if isinstance(i, dict):
             lm.update(i)

tests/test-command-template.t

@@ -2790,6 +2790,14 b' Test new-style inline templating:'
   $ hg log -R latesttag -r tip --template 'modified files: {file_mods % " {file}\n"}\n'
   modified files:  .hgtags
   
+
+  $ hg log -R latesttag -r tip -T '{rev % "a"}\n'
+  hg: parse error: keyword 'rev' is not iterable
+  [255]
+  $ hg log -R latesttag -r tip -T '{get(extras, "unknown") % "a"}\n'
+  hg: parse error: None is not iterable
+  [255]
+
 Test the sub function of templating for expansion:
 
   $ hg log -R latesttag -r 10 --template '{sub("[0-9]", "x", "{rev}")}\n'