PlugUpdate fails sometimes over SSH: "Could not read from remote repository"

I have configured git to use SSH instead of HTTP when connecting to Github:

```
# in my ~/.gitconig
[url "ssh://git@github.com/"]
insteadOf = https://github.com/
```

I use `gpg-agent` to remember my private SSH key for a certain amount of hours, so I only have to write my passphrase once a day to access my SSH key:

```
; # I can do this without having to write my passphrase again and again
; ssh -T git@github.com
Hi alcortesm! You've successfully authenticated, but GitHub does not provide shell access.
```

But when I tell vim-plug to check for updates I get inconsistent behaviour:

- some of my plugins are updated correctly
- some of them fail to update with this message:
```
fatal: Could not read from remote repository.

Please make sure you have the correct access rights and the repository exists.
```
- some of them pop up a `pinentry` window to ask me for my SSH passphrase even though my gpg-agent already knows the SSH private key.

Every time I try to PlugUpdate, different plugins show different behaviour from the list above and I haven't been able to find any patterns. I use 12 plugins, every time I try to update them I get 1-3 errors and 1-2 pinentry windows popups.

The problem only happens if I configure git to use SSH instead of HTTP, if I disable that, all plugins are correctly updated.

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

```
VIM - Vi IMproved 8.1 (2018 May 18, compiled Jun 15 2019 16:41:15)
Included patches: 1-875, 878, 884, 948, 1046, 1365-1368, 1382, 1401
Modified by team+vim@tracker.debian.org
Compiled by team+vim@tracker.debian.org
Huge version with GTK2 GUI. Features included (+) or not (-):
+acl +extra_search +mouse_netterm +tag_old_static
+arabic +farsi +mouse_sgr -tag_any_white
+autocmd +file_in_path -mouse_sysmouse +tcl
+autochdir +find_in_path +mouse_urxvt +termguicolors
-autoservername +float +mouse_xterm +terminal
+balloon_eval +folding +multi_byte +terminfo
+balloon_eval_term -footer +multi_lang +termresponse
+browse +fork() -mzscheme +textobjects
++builtin_terms +gettext +netbeans_intg +textprop
+byte_offset -hangul_input +num64 +timers
+channel +iconv +packages +title
+cindent +insert_expand +path_extra +toolbar
+clientserver +job +perl +user_commands
+clipboard +jumplist +persistent_undo +vartabs
+cmdline_compl +keymap +postscript +vertsplit
+cmdline_hist +lambda +printer +virtualedit
+cmdline_info +langmap +profile +visual
+comments +libcall -python +visualextra
+conceal +linebreak +python3 +viminfo
+cryptv +lispindent +quickfix +vreplace
+cscope +listcmds +reltime +wildignore
+cursorbind +localmap +rightleft +wildmenu
+cursorshape +lua +ruby +windows
+dialog_con_gui +menu +scrollbind +writebackup
+diff +mksession +signs +X11
+digraphs +modify_fname +smartindent -xfontset
+dnd +mouse +startuptime +xim
-ebcdic +mouseshape +statusline +xpm
+emacs_tags +mouse_dec -sun_workshop +xsmp_interact
+eval +mouse_gpm +syntax +xterm_clipboard
+ex_extra -mouse_jsbterm +tag_binary -xterm_save
system vimrc file: "$VIM/vimrc"
user vimrc file: "$HOME/.vimrc"
2nd user vimrc file: "~/.vim/vimrc"
user exrc file: "$HOME/.exrc"
system gvimrc file: "$VIM/gvimrc"
user gvimrc file: "$HOME/.gvimrc"
2nd user gvimrc file: "~/.vim/gvimrc"
defaults file: "$VIMRUNTIME/defaults.vim"
system menu file: "$VIMRUNTIME/menu.vim"
fall-back for $VIM: "/usr/share/vim"
Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H -DFEAT_GUI_GTK -pthread -I/usr/include/gtk-2.0 -I/usr/lib/x86_64-linux-gnu/gtk-2.0/include -I/usr/inEvery time I update the plugins each plugin show
clude/gio-unix-2.0 -I/usr/include/cairo -I/usr/include/pango-1.0 -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pixman-1 -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/pango-1.0 -I/usr/include/harfbuzz -I/usr/include/pango-1Every time I update the plugins each plugin show
.0 -I/usr/include/fribidi -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/uuid -I/usr/include/freetype2 -I/usr/include/libpng16 -Wdate-time -g -O2 -fdebug-prefix-map=/build/vim-4Pursk/vim-8.1.0875=. -fstack-protector-strong -Wformat -Werror=format-security -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1
Linking: gcc -L. -Wl,-z,relro -Wl,-z,now -fstack-protector -rdynamic -Wl,-export-dynamic -Wl,-E -Wl,-z,relro -Wl,-z,now -Wl,--as-needed -o vim -lgtk-x11-2.0 -lgdk-x11-2.0 -lpangocairo-1.0 -latk-1.0 -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lpangoft2-1.0 -lpango-1.0 -lgobject-2.0 -lglib-2.0 -lfontconfig -lfreetype -lSM -lICE -lXpm -lXt -lX11 -lXdmcp -lSM -lICE -lm -ltinfo -lnsl -lselinux -lacl -lattr -lgpm -ldl -L/usr/lib -llua5.2 -Wl,-E -fstack-protector-strong -L/usr/local/lib -L/usr/lib/x86_64-linux-gnu/perl/5.28/CORE -lperl -ldl -lm -lpthread -lcrypt -L/usr/lib/python3.7/config-3.7m-x86_64-linux-gnu -lpython3.7m -lcrypt -lpthread -ldl -lutil -lm -L/usr/lib/x86_64-linux-gnu -ltcl8.6 -ldl -lz -lpthread -lm -lruby-2.5 -lpthread -lgmp -ldl -lcrypt -lm

```

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