My Git Workflow

The illustration shows that “git add” is the only (everyday) operation that
can cause the index to diverge from the local repository. The only reason (in
Subversion-emulation mode) to use “git add” is so that “git commit” will see
your changes. The -a option to “git commit” causes “git commit” to run
“git add -u” first – in which case you never need to run "git add -u”
explicitly – in which case the index stays in sync with the repository head.