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