githelp: translate --directory of git apply to --prefix

According to the help pages, these flags do the same.

Differential Revision: https://phab.mercurial-scm.org/D6459

hgext/githelp.py

@@ -192,12 +192,15 b' def am(ui, repo, *args, **kwargs):'
 def apply(ui, repo, *args, **kwargs):
     cmdoptions = [
         ('p', 'p', int, ''),
+        ('', 'directory', '', ''),
     ]
     args, opts = parseoptions(ui, cmdoptions, args)
 
     cmd = Command('import --no-commit')
     if (opts.get('p')):
         cmd['-p'] = opts.get('p')
+    if opts.get('directory'):
+        cmd['--prefix'] = opts.get('directory')
     cmd.extend(args)
 
     ui.status((bytes(cmd)), "\n")

tests/test-githelp.t

@@ -256,6 +256,10 b' githelp for apply with directory strip c'
   $ hg githelp -- apply -p 5
   hg import --no-commit -p 5
 
+githelp for apply with prefix directory
+  $ hg githelp -- apply --directory=modules
+  hg import --no-commit --prefix modules
+
 git merge-base
   $ hg githelp -- git merge-base --is-ancestor
   ignoring unknown option --is-ancestor