branch: --no-optional-locks arg needs to be given to git

The special argument --no-optional-lock applies to the main git command,
and not to the git subcommand `git status`. So move it there, to prevent
an error with git.

closes #2651

Signed-off-by: Christian Brabandt <cb@256bit.org>

autoload/airline/extensions/branch.vim

@@ -17,8 +17,8 @@ scriptencoding utf-8
 let s:vcs_config = {
 \  'git': {
 \    'exe': 'git',
-\    'cmd': 'git status --porcelain --no-optional-locks -- ',
-\    'dirty': 'git status -uno --porcelain --no-optional-locks --ignore-submodules',
+\    'cmd': 'git --no-optional-locks status --porcelain -- ',
+\    'dirty': 'git --no-optional-locks status -uno --porcelain --ignore-submodules',
 \    'untracked_mark': '??',
 \    'exclude': '\.git',
 \    'update_branch': 's:update_git_branch',