Wildcard tag doesn't play well with `:PlugUpdate` when tags are detached ones.

(I was under the impression that this plugin supports wildcard tag releases properly, so correct me if I am wrong.)

`:PlugUpdate` seems won't fetch latest tags when the plugin has property of wildcard tag.

With this minimal vimrc
```
call plug#begin('~/.vim/plugged')
Plug 'autozimu/plug-wildcard', {'tag': 'release-*'}
call plug#end()
```

1. Run `:PlugInstall`. This will install the plugin properly.
2. Make a newer release in the repo, say `release-0.2`. The tag is slightly irregular. It works like this
```sh
# make changes
git commit -m 'new release'
git push --tags
```
(Note at this point, only tags are pushed. Current branch HEAD is not pushed.)
3. Run `:PlugUpdate`.

At this point, the plugin is not updated, while it is expected to update to `release-0.2`.

(This affects https://github.com/autozimu/LanguageClient-neovim/issues/239)

------------------------------

Neovim version
```
NVIM v0.2.2
Build type: Release
LuaJIT 2.0.5
Compilation: /usr/bin/cc -Wconversion -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -O2 -DNDEBUG -DMIN_LOG_LEVEL=3 -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wvla -fstack-protector --param ssp-buffer-size=4 -Wno-array-bounds -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -I/opt/neovim/build/config -I/opt/neovim/src -I/opt/neovim/.deps/usr/include -I/opt/neovim/.deps/usr/include -I/opt/neovim/.deps/usr/include -I/opt/neovim/.deps/usr/include -I/opt/neovim/.deps/usr/include -I/opt/neovim/.deps/usr/include -I/nix/store/s96g9b8rl9ab13v1by7n661h0hzpbgsw-glibc-2.25-49-dev/include -I/opt/neovim/build/src/nvim/auto -I/opt/neovim/build/include

Features: +acl +iconv +jemalloc +tui
See ":help feature-compile"

system vimrc file: "$VIM/sysinit.vim"
fall-back for $VIM: "/usr/local/share/nvim"

Run :checkhealth for more info
```

- Type:
- [x] Bug
- [ ] Enhancement
- [ ] Feature Request
- [ ] Question
- OS:
- [ ] All/Other
- [x] Linux
- [ ] OS X
- [ ] Windows
- Vim:
- [ ] Terminal Vim
- [ ] GVim
- [x] Neovim

Edit: only detached tags can reproduce this issue.