Error with `dein#update()` when specifying a revision number.

[hokorobi]

Problems summary

I get an error with dein#update() when specifying a revision number. Specifing a branch, it is not an error.

Expected

The error doesn't happen. But, a response of "Use frozen" might be fine.

Environment Information (Required!)

• dein.vim version(SHA1): 47d66aa (e570f69 is not an error)

• OS: Windows10

• Vim/neovim version: Vim 8.2.1512

Provide a minimal .vimrc with less than 50 lines (Required!)

set nocompatible
set runtimepath+=C:\Users\hokorobi\_vim\dein\repos\github.com\Shougo\dein.vim
call dein#begin('~/_vim/dein/.cachetest')
call dein#add('~/_vim/dein/repos/github.com/Shougo/dein.vim')
call dein#add('tsuyoshicho/plantuml-previewer.vim', {'rev': '1b2f9c6'})
call dein#end()
filetype plugin indent on
syntax enable

The reproduce ways from Vim starting (Required!)

1. vim -N -u ~/vimfiles/minimal-vimrc -U NONE -i NONE
2. call dein#install()
3. call dein#update()

Upload the log messages by :redir and :message (if errored)

[dein] Update started: (2020/08/23 12:00:27)
[dein] C:/Users/hokorobi/_vim/dein/.cachetest/repos/github.com/tsuyoshicho/plantuml-previewer.vim_1b2f9c6
[dein] You are not currently on a branch.
[dein] Please specify which branch you want to merge with.
[dein] See git-pull(1) for details.
[dein]     git pull <remote> <branch>
[dein] You are not currently on a branch.
[dein] Please specify which branch you want to merge with.
[dein] See git-pull(1) for details.
[dein]     git pull <remote> <branch>
[dein] Error installing plugins:
[dein]   plantuml-previewer.vim
[dein] Please read the error message log with the :message command.
[dein] Done: (2020/08/23 12:00:29)

[hokorobi]

Thank you!