git add . vs git commit -a
git commit -a means almost[*] the same thing as git add -u && git commit.
It's not the same as git add . as this would add untracked files that aren't being ignored, git add -u only stages changes (including deletions) to already tracked files.
[*] There's a subtle difference if you're not at the root directory of your repository. git add -u stages updates to files in the current directory and below, it's equivalent to git add -u . whereas git commit -a stages and commits changes to all tracked files.
git commit -a automatically invokes git add on all files it knows about. You can use git add to select what files to commit. Consult the docs for more info: here
By using the git commit -a switch with the commit command to automatically "add" changes from all known files (i.e. all files that are already listed in the index)