rcutil: add a method to convert environment variables to config items

Handling config and environ priorities has been messy. Partially because we
don't have config layers - you either get all configs (sys + user), or none.

Ideally, environ like $EDITOR, $PAGER should be able to override the system
configs "ui.editor", "pager.pager". This patch provides the ability to
convert them into config items, so they can be inserted into the middle
config layer between system rc and user rc.

mercurial/rcutil.py

@@ -32,6 +32,28 b' def _expandrcpath(path):'
         return [join(p, f) for f, k in osutil.listdir(p) if f.endswith('.rc')]
     return [p]
 
+def envrcitems(env=None):
+    '''Return [(section, name, value, source)] config items.
+
+    The config items are extracted from environment variables specified by env,
+    used to override systemrc, but not userrc.
+
+    If env is not provided, encoding.environ will be used.
+    '''
+    if env is None:
+        env = encoding.environ
+    checklist = [
+        ('EDITOR', 'ui', 'editor'),
+        ('VISUAL', 'ui', 'editor'),
+        ('PAGER', 'pager', 'pager'),
+    ]
+    result = []
+    for envname, section, configname in checklist:
+        if envname not in env:
+            continue
+        result.append((section, configname, env[envname], '$%s' % envname))
+    return result
+
 def defaultrcpath():
     '''return rc paths in default.d'''
     path = []