git-gui: Relabel the Add All action.

git-gui: Relabel the Add All action.

One user that I spoke with recently was confused why the 'Add All'
button did not add all of his 'Changed But Not Updated' files.
The particular files in question were new, and thus not known to
Git.  Since the 'Add All' routine only updates files which are
already tracked, they were not added automatically.

I suspect that calling this action 'Add Existing' would be less
confusing, so I'm renaming it.

Signed-off-by: Shawn O. Pearce <redacted>