gitk: Add Brazilian Portuguese (pt-BR) translation

Translating a SCM is tricky due to amount of jargon, so, I tried to
keep the wording consistent with both the German and Italian git
translations and the pt-BR translation of other SCMs.

Signed-off-by: Alexandre Erwin Ittner <alexandre@ittner.com.br>
Signed-off-by: Paul Mackerras <paulus@samba.org>