messages: replace some instances of "folder" by "directory"

I'm pretty sure this is our preferred term.

Differential Revision: https://phab.mercurial-scm.org/D6222

mercurial/context.py

@@ -1965,7 +1965,7 b' class overlayworkingctx(committablectx):'
             if not mfiles:
                 return
             raise error.Abort("error: file '%s' cannot be written because "
-                              " '%s/' is a folder in %s (containing %d "
+                              " '%s/' is a directory in %s (containing %d "
                               "entries: %s)"
                               % (path, path, self.p1(), len(mfiles),
                                  ', '.join(mfiles)))

mercurial/debugcommands.py

@@ -1143,7 +1143,7 b' def debugignore(ui, repo, *files, **opts'
                     ui.write(_("%s is ignored\n") % uipathfn(f))
                 else:
                     ui.write(_("%s is ignored because of "
-                               "containing folder %s\n")
+                               "containing directory %s\n")
                              % (uipathfn(f), ignored))
                 ignorefile, lineno, line = ignoredata
                 ui.write(_("(ignore rule in %s, line %d: '%s')\n")

tests/test-rebase-inmemory.t

@@ -252,7 +252,7 b' Test reporting of path conflicts'
   $ hg rebase -r 3 -d . -n
   starting dry-run rebase; repository will not be changed
   rebasing 3:844a7de3e617 "c"
-  abort: error: file 'c' cannot be written because  'c/' is a folder in e147e6e3c490 (containing 1 entries: c/subdir/file.txt)
+  abort: error: file 'c' cannot be written because  'c/' is a directory in e147e6e3c490 (containing 1 entries: c/subdir/file.txt)
   [255]
 
   $ cd ..