Fix wrapper slowness in vcs segment

[guicassolato]

This prevents git command from passing through eventual aliases, commonly defined when Git wrappers (e.g. hub) are used.

[dritter]

Thanks @guicassolato for this PR (and congrats for the 400th PR ;) ).
I think this PR deserves a bit more explanation.

From the docs:

command [ -pvV ] simple command
The simple command argument is taken as an external command instead of a function or builtin and is executed. If the POSIX_BUILTINS option is set, builtins will also be executed but certain special properties of them are suppressed. The -p flag causes a default path to be searched instead of that in $path. With the -v flag, command is similar to whence and with -V, it is equivalent to whence -v.

So the command builtin skips possible wrapper scripts and executes the command directly. If I have an alias for git (e.g. alias git='echo "git-wrapper"; git'; stupid, I know) and I execute git status in a repository, I'll get git-wrapper besides the git status output. But if I do command git status I'll get only the output of that command (without git-wrapper).

[bhilburn]

This is excellent, @guicassolato! This will certainly improve a number of issues, including extremely confusing bug reports as well as likely speeding some things up. And thanks, @dritter, for explaining how it works!

We're right in the middle of a major code re-org in #711, which will affect where this changeset is applied (although not the actual logic of the code itself). To avoid merge conflicts, I'm going to wait until that PR is done (should be shortly), and then we can update this PR and merge =)