Fixes git_compare_version() in lib/git.zsh and refactors the whole file

[LFDM]

git_compare_version() is used to determine whether dirty submodules should be ignored or not, but git has to be more recent than version 1.7.2 to know about that option.

With the former comparison logic a check if git version 1.8.1 is more recent than 1.7.2 turned out negative, which is fixed through this pull request.

UPDATE: In addition to #2401 I refactored the whole file.

• Cleans up it's functions
• Less duplication of code and logic
• Less querying of git and the general status,
• Adds functions that where buried inside of others before (git_stash_status, git_change_status) to gain better modularity.
• Adds a big documentation section where every function and customization variable is explained.
• Adds a function to hide/show the git prompt in specific large repositories - toggle_git_prompt_visibility

Might need some more testing if anything was broken, though I don't have any troubles locally.

Possibly breaking changes:

• Cuts the duplicated variables to show the git_remote_status.

[LFDM]

Could also fix #2404, if people approve.

[bric3]

I'm using my own version of oh-my-git now which doesn't rely on oh-my-zsh git functions, so I didn't yet test it, but it seems ok by just reading the code. However I think proper tests need to be done.

[robbyrussell]

We need some testers!

[simonbuchan]

@robbyrussell As this resolves my #2928, I'm going to be running this, but I'm not exactly a tester.

Merge seems to resolve fine taking all from this (git.zsh conflicts look like they're all done here too)

[simonbuchan]

My merge in simonbuchan/oh-my-zsh pull-2386

[simonbuchan]

@LFDM This seems to be missing git_stash_status, unless my merge broke it?

[simonbuchan]

Updated my merge, looks like I missed some stuff last try.

Looks like git_prompt_status was using current_branch instead of git_current_branch?

Might want to get new git_{prompt,commits}_ahead into git_prompt_status somehow?