pathcomplete: complete directories more conservatively

Suppose we want to perform a single-level completion (i.e. without
--full) of "fi" in a repo containing "fee", "fie/dead", "fie/live",
and "foe".

If we give back "fie/" as the only answer, the shell will consider
the completion to be unambiguous, and will append a space after the
completion.  We can't complete "fie/live" or "fie/dead" without
first backspacing over that space.

We used to thus create two fake names, "fie/a" and "fie/b", to force
the shell to consider the completion to be ambiguous. It would then
stop at "fie/" without appending a space, allowing us to hit tab
again to complete "fie/live" or "fie/dead".

The change here arises from realising that we only need to force
the shell to consider a completion as ambiguous if we have exactly
one directory and zero files as possible completions.

This prevents spurious names from showing up as possible completions
when they don't need to be invented in the first place.

mercurial/commands.py

@@ -2198,9 +2198,11 b' def debugpathcomplete(ui, repo, *specs, '
         f, d = complete(spec, acceptable or 'nmar')
         files.update(f)
         dirs.update(d)
-    for d in dirs:
-        files.add(d + 'a')
-        files.add(d + 'b')
+    if not files and len(dirs) == 1:
+        # force the shell to consider a completion that matches one
+        # directory and zero files to be ambiguous
+        dirs.add(iter(dirs).next() + '.')
+    files.update(dirs)
     ui.write('\n'.join(repo.pathto(p, cwd) for p in sorted(files)))
     ui.write('\n')