Add Git tags to prompt

Example:
  ➜  abc git:(v1.0 @ f00baa)
  ➜  buz git:(v0_5-rc1 some-tag @ a1b2c3)

If using git >= 1.7.10, will show up to 10 tags separated by spaces
For earlier versions, only single (last) tag will be shown.

Minimum supported Git version: v1.0.5

External tool dependencies - head(1), paste(1)

Closes robbyrussell/oh-my-zsh#1599

Signed-off-by: Aleksandrs Ļedovskis <aleksandrs@ledovskis.lv>