git-summary: protect against character encoding issues with LC_ALL=C

[apjanke]

On OS X 10.9.5, I get a couple sed errors in git-summary-line.

These look like character encoding issues.

[~/local/opp/omz/oh-my-zsh on ⇄ git-extras-compatibility]
$ git-summary --line

 project  : oh-my-zsh
sed: RE error: illegal byte sequence
sed: RE error: illegal byte sequence
 lines    : 34168
 authors  :
 3938 Andrew Janke              11.5%
 3374 Felipe Contreras          9.9%

Running sed under LC_ALL=C makes the errors go away. This PR does that.

[hemanth]

👍