diff: support diffing explicit files in subrepos

Most other commands support implied recursion based on file names already.

mercurial/help/subrepos.txt

@@ -105,8 +105,10 b' Interaction with Mercurial Commands'
     Subversion subrepositories will print a warning and abort.
 
 :diff: diff does not recurse in subrepos unless -S/--subrepos is
-    specified. Changes are displayed as usual, on the subrepositories
-    elements. Subversion subrepositories are currently silently ignored.
+    specified.  However, if you specify the full path of a file or
+    directory in a subrepo, it will be diffed even without
+    -S/--subrepos being specified.  Subversion subrepositories are
+    currently silently ignored.
 
 :files: files does not recurse into subrepos unless -S/--subrepos is
     specified.  However, if you specify the full path of a file or

mercurial/logcmdutil.py

@@ -129,7 +129,6 b' def diffordiffstat(ui, repo, diffopts, n'
             for chunk, label in chunks:
                 ui.write(chunk, label=label)
 
-    if listsubrepos:
         for subpath, sub in scmutil.itersubrepos(ctx1, ctx2):
             tempnode2 = node2
             try:
@@ -142,6 +141,7 b' def diffordiffstat(ui, repo, diffopts, n'
                 tempnode2 = None
             submatch = matchmod.subdirmatcher(subpath, match)
             subprefix = repo.wvfs.reljoin(prefix, subpath)
+        if listsubrepos or match.exact(subpath) or any(submatch.files()):
             sub.diff(ui, diffopts, tempnode2, submatch, changes=changes,
                      stat=stat, fp=fp, prefix=subprefix)
 

tests/test-subrepo.t

@@ -1875,6 +1875,19 b" Test that '[paths]' is configured correc"
   @@ -0,0 +1,1 @@
   +bar
 
+  $ hg diff -X '.hgsub*' --nodates s
+  diff -r 000000000000 s/a
+  --- /dev/null
+  +++ b/s/a
+  @@ -0,0 +1,1 @@
+  +a
+  $ hg diff -X '.hgsub*' --nodates s/a
+  diff -r 000000000000 s/a
+  --- /dev/null
+  +++ b/s/a
+  @@ -0,0 +1,1 @@
+  +a
+
   $ cd ..
 
 test for ssh exploit 2017-07-25