Support wildcards in tag option

Close #427

plug.vim

@@ -28,6 +28,9 @@
 "   " Using a non-master branch
 "   Plug 'rdnetto/YCM-Generator', { 'branch': 'stable' }
 "
+"   " Using a tagged release; wildcard allowed (requires git 1.9.2 or above)
+"   Plug 'fatih/vim-go', { 'tag': '*' }
+"
 "   " Plugin options
 "   Plug 'nsf/gocode', { 'tag': 'v.20150303', 'rtp': 'vim' }
 "
@@ -966,8 +969,17 @@ function! s:update_finish()
         call s:log4(name, 'Checking out '.spec.commit)
         let out = s:checkout(spec)
       elseif has_key(spec, 'tag')
-        call s:log4(name, 'Checking out '.spec.tag)
-        let out = s:system('git checkout -q '.s:esc(spec.tag).' 2>&1', spec.dir)
+        let tag = spec.tag
+        if tag =~ '\*'
+          let tags = s:lines(s:system('git tag --list '.string(tag).' --sort -version:refname 2>&1', spec.dir))
+          if !v:shell_error && !empty(tags)
+            let tag = tags[0]
+            call s:log4(name, printf('Latest tag for %s -> %s', spec.tag, tag))
+            call append(3, '')
+          endif
+        endif
+        call s:log4(name, 'Checking out '.tag)
+        let out = s:system('git checkout -q '.s:esc(tag).' 2>&1', spec.dir)
       else
         let branch = s:esc(get(spec, 'branch', 'master'))
         call s:log4(name, 'Merging origin/'.branch)
@@ -2227,4 +2239,3 @@ endif
 
 let &cpo = s:cpo_save
 unlet s:cpo_save
-