* Ensure options for formulas `git.latest` are also based on defaults

* Prevent erroneous remote tracking branch modification by `salt.formulas` #238

* Allow multiple envs to use the same gitdir for `salt.formulas`

* Ensure `gitdir` original branch is checked out after formulas `git.latest`

* Fix erroneous comment re: merging in `salt` pillar

This restores the original structure/behavior of the
`salt_formulas` Pillar key.

Jinja macros are not actually functions.  The only thing they can return
is a string.  In order to return structured data, the callee must
serialize it, and the caller must deserialize it.  This state formula
uses YAML as the intermediary, hence the occurrence of both the
`|yaml` (callee) and `|load_yaml` (caller) filters in its code.

The post-render "mapping values are not allowed here" error in
*salt/formulas.sls* or the broken rendering of
*salt/files/master.d/f_defaults.conf* happens because invocations of the
`formulas_git_opt` macro in several Jinja `set` statements do not get
deserialized, resulting in the trailing newline followed by three dot
characters (`...`), which YAML uses to signal the end of a document.
Correcting these rendering errors requires adding the necessary
deserialization code at those locations (i.e., filtering the macro call
through `|load_yaml`).

This test isn't available in certain popular versions of Jinja (namely the one
installed by default in CentOS 6).

This state downloads formulas from git repositories and ensures
that they are recorded in the file_roots setting.