Add functions for working with git repository

[VxDxK]

It can be easily added via libgit to avoid boilerplate scripts like:

# Instead of
@cur-branch:
    echo Current branch is $(git branch 2> /dev/null | sed -e '/^[^*]/d' -e 's/* \(.*\)/\1/')

# Will possible write
@cur-branch:
    echo Current branch is {{git_branch()}}

I suggest next functions:

1. git_repo - (path to root of current repo)
2. git_branch - (current branch)
3. git_revision - (hash of latest commit)
4. git_tag - (latest tag)
5. git_is_dirty - (is current branch dirty)

[casey]

These are all pretty simple:

repo: git rev-parse --show-toplevel
branch: git branch --show-current
commit: git rev-parse HEAD
tag: git describe --tags --abbrev=0
dirty or not: git diff --quiet || echo dirty

I don't think it's worth adding a dependency for this, since git2 is pretty heavyweight.