git_config: handle configuration entries with no values

A git-config entry with no value was preventing repo
from initializing.  This modifies _ReadGit() to handle
config entries with empty values.

Signed-off-by: David Aguilar <davvid@gmail.com>
Reported-by: Josh Guilfoyle <jasta00@gmail.com>

tests/fixtures/test.gitconfig

@@ -0,0 +1,3 @@
+[section]
+	empty
+	nonempty = true