[PATCH 2 of 4] templater: add wrapper for a single template mapping

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1539951090 -32400
#      Fri Oct 19 21:11:30 2018 +0900
# Node ID 49a6ece003a203da3e4a74fb60256f811fd2f32b
# Parent  276558285cd47fc388aed7861eaf7237c6d184e3
templater: add wrapper for a single template mapping

This can be used to nest template mappings without inserting a sequence-like
layer. See the next patch for example.

diff --git a/mercurial/templater.py b/mercurial/templater.py
--- a/mercurial/templater.py
+++ b/mercurial/templater.py
@@ -49,6 +49,10 @@ mappinggenerator, mappinglist
     represents mappings (i.e. a list of dicts), which may have default
     output format.
 
+mappingdict
+    represents a single mapping (i.e. a dict), which may have default output
+    format.
+
 mappedgenerator
     a lazily-evaluated list of byte strings, which is e.g. a result of %
     operation.
diff --git a/mercurial/templateutil.py b/mercurial/templateutil.py
--- a/mercurial/templateutil.py
+++ b/mercurial/templateutil.py
@@ -472,6 +472,29 @@ class mappinglist(_mappingsequence):
     def tobool(self, context, mapping):
         return bool(self._mappings)
 
+class mappingdict(mappable, _mappingsequence):
+    """Wrapper for a single template mapping
+
+    This isn't a sequence in a way that the underlying dict won't be iterated
+    as a dict, but shares most of the _mappingsequence functions.
+    """
+
+    def __init__(self, mapping, name=None, tmpl=None):
+        super(mappingdict, self).__init__(name, tmpl)
+        self._mapping = mapping
+
+    def tomap(self, context):
+        return self._mapping
+
+    def tobool(self, context, mapping):
+        # no idea when a template mapping should be considered an empty, but
+        # a mapping dict should have at least one item in practice, so always
+        # mark this as non-empty.
+        return True
+
+    def tovalue(self, context, mapping):
+        return super(mappingdict, self).tovalue(context, mapping)[0]
+
 class mappedgenerator(wrapped):
     """Wrapper for generator of strings which acts as a list