Change all whoami calls for $USER variable

This avoid spawning additional processes, as the $USER variable will
**always** contain the current logged-in user.
Change all pwd calls for $PWD variable

This avoids spawning additional processes as $PWD **always** contains
the current working directory.
Change all hostname calls for $HOST or $SHORT_HOST
Merge pull request #3223 from mcornella/use-special-vars

Use special variables where we can
Add submodule helpers to Git plugin

From #1116

Modified: compdef needs _git entry point
Modifier: ncanceill <https://github.com/ncanceill>

Original commit:
Author :     Maxime Fabre <ehtnam6@gmail.com>

diff --git a/plugins/git/git.plugin.zsh b/plugins/git/git.plugin.zsh
index 6e6ba05..53046de 100644
--- a/plugins/git/git.plugin.zsh
+++ b/plugins/git/git.plugin.zsh
@@ -161,4 +161,10 @@ alias gunignore='git update-index --no-assume-unchanged'
 alias gignored='git ls-files -v | grep "^[[:lower:]]"'

-
+# Submodules
+alias gf='git fetch'
+compdef _git gf=git-fetch
+alias gsi='git submodule init'
+compdef gsi=git-submodule
+alias gsu='git submodule update'
+compdef gsu=git-submodule
Additional commands to git plugin

Original author :     Kirill Panshin <kipanshi@gmail.com>
Merged & cleaned by:  ncanceill <https://github.com/ncanceill>
Add aliases for `git rebase` and `git rebase skip`

Original author :     Marten Lienen <marten.lienen@gmail.com>
Merged & cleaned by:  ncanceill <https://github.com/ncanceill>
fix gwip alias for some BSD flavors, fixes #2692
plugins/git: missing compdefs, fix bad copy-paste

Original author :     Hervé Cauwelier <herve.cauwelier@polyconseil.fr>
Merged & cleaned by:  ncanceill <https://github.com/ncanceill>
Fix gmt compdef

Fix typo: gm -> gmt
add alias for 'git stash list'
re-organized plugin (no real change)
update aliases to default to current branch, fixes #1601
improve readme to abide by #2640
change alias gwc to gwch, fixes #2463
add aliases from #2494
add gbnm alias for non-merged branches
inspired from #1982
bring back `git ls-files|grep $1` as a function, and add completion
Avoid showing 'git rm' usage when using gwip alias
Added aliases suggested by @santacruz123
fixes #2433