Interpret the second argument to Plug command as tag option

The older documentation of vim-plug suggested that the second string
parameter to Plug command was for both branches and tags. But we've
realized that tags and branches are not interchangeable (#174), and it
is now recommended that the user explicitly specify whether it's a
branch or a tag. The now-undocumented second parameter is currently for
branches, but I'd like to change it for tags, as the use of tags are
much more common.

plug.vim

@@ -439,12 +439,9 @@ function! s:parse_options(arg)
   let opts = copy(s:base_spec)
   let type = type(a:arg)
   if type == s:TYPE.string
-    let opts.branch = a:arg
+    let opts.tag = a:arg
   elseif type == s:TYPE.dict
     call extend(opts, a:arg)
-    if has_key(opts, 'tag')
-      let opts.tag = remove(opts, 'tag')
-    endif
     if has_key(opts, 'dir')
       let opts.dir = s:dirpath(expand(opts.dir))
     endif

test/regressions.vader

@@ -100,7 +100,7 @@ Execute (#139-1 Using new remote branch):
   " We're setting up two plugins so that parallel installer is used
   call plug#begin('$TMPDIR/plugged')
   Plug 'junegunn/seoul256.vim'
-  Plug expand('file:///$TMPDIR/new-branch'), 'new'
+  Plug expand('file:///$TMPDIR/new-branch'), { 'branch': 'new' }
   call plug#end()
   PlugUpdate
   silent %y
@@ -130,7 +130,7 @@ Execute (#139-2 Using yet another new remote branch):
 
   " Test Vim installer here
   call plug#begin('$TMPDIR/plugged')
-  Plug expand('file:///$TMPDIR/new-branch'), 'brand-new'
+  Plug expand('file:///$TMPDIR/new-branch'), { 'branch': 'brand-new' }
   call plug#end()
   PlugUpdate
   silent %y

test/workflow.vader

@@ -33,7 +33,7 @@ Execute (Subsequent plug#begin() calls will reuse g:plug_home):
 
 Execute (Test Plug command):
 ^ Git repo with branch (DEPRECATED. USE BRANCH OPTION)
-  Plug 'junegunn/seoul256.vim', 'yes-t_co'
+  Plug 'junegunn/seoul256.vim', { 'branch': 'yes-t_co' }
   AssertEqual 'file:///tmp/junegunn/seoul256.vim', g:plugs['seoul256.vim'].uri
   AssertEqual join([temp_plugged, 'seoul256.vim/'], '/'), g:plugs['seoul256.vim'].dir
   AssertEqual 'yes-t_co', g:plugs['seoul256.vim'].branch
@@ -45,7 +45,7 @@ Execute (Test Plug command):
   Plug 'junegunn/goyo.vim', '1.5.2'
   AssertEqual 'file:///tmp/junegunn/goyo.vim', g:plugs['goyo.vim'].uri
   AssertEqual join([temp_plugged, 'goyo.vim/'], '/'), g:plugs['goyo.vim'].dir
-  AssertEqual '1.5.2', g:plugs['goyo.vim'].branch
+  AssertEqual '1.5.2', g:plugs['goyo.vim'].tag
 
   Plug 'junegunn/goyo.vim', { 'tag': '1.5.3' } " Using tag option
   AssertEqual '1.5.3', g:plugs['goyo.vim'].tag