packaging: rename git.svg to git-cola.svg

Fixup the DESTDIR handling in the Makefile and tidy things up.

Closes #359
Suggested-by: Ｖ字龍(Vdragon) <Vdragon.Taiwan@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
doc: add v.paritskiy to the credits

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge branch 'icon-fixups'

* icon-fixups:
  doc: add v.paritskiy to the credits
  packaging: rename git.svg to git-cola.svg

Closes #359
Signed-off-by: David Aguilar <davvid@gmail.com>