Made buildinfo aware of builds from GIT

- Use commit number since last annotated tag as a
  revision number replacement. It'll eb followed
  by 'M' symbol if there're local modification in
  the source tree.

- Commit short SHA1 is included. Helps getting
  information about commit used to build blender
  with much faster.

- If build is not done from master branch, this also
  will be noticed in the splash screen.

This commit also replaces revision stored in the
files with git-specific fields (change and hash).
This is kind of breaks compatibility, meaning
files which were saved before this change wouldn't
display any information about which revision they
were saved with. When we'll finally switch to git,
we'll see proper hash and change number since
previous release in the files, for until then
svn version will be used as a change number and
hash will be empty.

Not a huge deal, since this field was only used
by developers to help torubleshooting things and
isn't needed for blender itself.

Some additional tweaks are probably needed :)

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61082 954f8c5b-7b00-dc11-b283-0030488c597c