templatekw: add {size} keyword as an example of fctx-based keyword

I'll add {status}, and I think some lfs keywords can be migrated to this.
I'm not certain how many fctx-based keywords will be introduced into the
global space, but if there are a couple more, we'll probably need to sort
them out to the "File Keywords" section in the templater help. Until then,
fctx keywords are hidden as experimental.

mercurial/templatekw.py

@@ -582,6 +582,12 b' def showreporoot(context, mapping):'
     repo = context.resource(mapping, 'repo')
     return repo.root
 
+@templatekeyword('size', requires={'fctx'})
+def showsize(context, mapping):
+    """Integer. Size of the current file in bytes. (EXPERIMENTAL)"""
+    fctx = context.resource(mapping, 'fctx')
+    return fctx.size()
+
 @templatekeyword("successorssets", requires={'repo', 'ctx'})
 def showsuccessorssets(context, mapping):
     """Returns a string of sets of successors for a changectx. Format used

tests/test-template-keywords.t

@@ -787,6 +787,15 b' Test file copies dict:'
   $ hg log -r8 -C -T '{file_copies_switch % "{path} <- {source}\n"}'
   fourth <- second
 
+Test file attributes:
+
+  $ hg log -l1 -T '{files % "{pad(size, 3, left=True)} {path}\n"}'
+      a
+    0 b
+    7 fifth
+      fourth
+   13 third
+
 Test index keyword:
 
   $ hg log -l 2 -T '{index + 10}{files % " {index}:{file}"}\n'