templatekw: add {path} keyword to host documentation

It's hidden since I'm not sure if filectx templates can be a thing. The plan
is to add {status}, {size}, etc., which are usable within {files % ...}
context.

mercurial/templatekw.py

@@ -543,6 +543,12 def showobsolete(context, mapping):
         return 'obsolete'
     return ''
 
+@templatekeyword('path', requires={'fctx'})
+def showpath(context, mapping):
+    """String. Repository-absolute path of the current file. (EXPERIMENTAL)"""
+    fctx = context.resource(mapping, 'fctx')
+    return fctx.path()
+
 @templatekeyword('peerurls', requires={'repo'})
 def showpeerurls(context, mapping):
     """A dictionary of repository locations defined in the [paths] section