template: add a utility function to render template

Summary:
The added method takes an environment (`props`), and a program written in
the template language, execute the program, and pass output to `ui.write`.

The API is similar to the upstream [1]. But it avoids `ctx` intentionally.
Since the "environment" should be more flexible than having to have a `ctx`.

[1]: aa32940279

Reviewed By: mitrandir77

Differential Revision: D8221079

fbshipit-source-id: 94ee5c563f943330ef91966a4883c5733cc8dac9

mercurial/cmdutil.py

@@ -966,6 +966,26 @@ def isstdiofilename(pat):
     return not pat or pat == "-"
 
 
+def rendertemplate(ui, tmpl, props=None):
+    """Render tmpl written in the template language. props provides the
+    "environment" which the template program runs in.
+
+    Return the rendered string.
+
+    >>> import mercurial.ui as uimod
+    >>> rendertemplate(uimod.ui(), '{a} {b|json}', {'a': 'x', 'b': [3, None]})
+    'x [3, null]'
+    """
+    t = formatter.maketemplater(ui, tmpl, cache=templatekw.defaulttempl)
+    mapping = {"ui": ui, "templ": t}
+    if props:
+        if "ctx" in props:
+            mapping["revcache"] = {}
+        mapping.update(props)
+    mapping.update(templatekw.keywords)
+    return t.render(mapping)
+
+
 class _unclosablefile(object):
     def __init__(self, fp):
         self._fp = fp

tests/test-doctest.py

@@ -49,6 +49,7 @@ def testmod(name, optionflags=0, testtarget=None):
 
 testmod("mercurial.changegroup")
 testmod("mercurial.changelog")
+testmod("mercurial.cmdutil")
 testmod("mercurial.color")
 testmod("mercurial.config")
 testmod("mercurial.context")