Use porcelain format in git status for dirty check

The porcelain format is better than the short (-s) one because it gives
the same output regardless of git version and user settings [1].

In this particular case, we change it because the short format outputs
branch information in case the user has the status.branch setting
enabled (set to 1) [2]. Because of that, the output of the git status
will always have a line, which piped to `tail -n1' will be interpreted
as being dirty.

[1] See http://git-scm.com/docs/git-status#_porcelain_format
[2] More info at #3126

Fix #3126