templates: add filter to reverse list

The filter supports only lists because for lists, it’s straightforward to
implement. Reversing text doesn’t seem very useful and is hard to implement.
Reversing the bytes would break multi-bytes encodings. Reversing the code
points would break characters consisting of multiple code points. Reversing
graphemes is non-trivial without using a library not included in the standard
library.

mercurial/templatefilters.py

@@ -390,6 +390,14 b' def person(author):'
     return stringutil.person(author)
 
 
+@templatefilter(b'reverse')
+def reverse(list_):
+    """List. Reverses the order of list items."""
+    if isinstance(list_, list):
+        return templateutil.hybridlist(list_[::-1], name=b'item')
+    raise error.ParseError(_(b'not reversible'))
+
+
 @templatefilter(b'revescape', intype=bytes)
 def revescape(text):
     """Any text. Escapes all "special" characters, except @.

tests/test-template-functions.t

@@ -1718,4 +1718,19 b' read config options:'
   $ hg log -T "{config('templateconfig', 'knob', if(true, 'foo', 'bar'))}\n"
   foo
 
+reverse filter:
+
+  $ hg log -T "{'abc\ndef\nghi'|splitlines|reverse}\n"
+  ghi def abc
+
+  $ hg log -T "{'abc'|reverse}\n"
+  hg: parse error: not reversible
+  (incompatible use of template filter 'reverse')
+  [10]
+
+  $ hg log -T "{date|reverse}\n"
+  hg: parse error: not reversible
+  (template filter 'reverse' is not compatible with keyword 'date')
+  [10]
+
   $ cd ..