The file 'po/git.pot' is generated using the command 'make pot'
against git v1.7.9-209-gb6b3b (Update draft release notes to 1.7.10).
Since po/git.pot is tracked, remove the entry from .gitignore, and
not delete the file again when doing 'make distclean'.
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>