[DOCBUG] git subtree synopsis needs updating

[Yann Dirson <dirson@bertin.fr>]

As the examples in git-subtree.txt show, the synopsis in the same file should
surely get a patch along the lines of:

-'git subtree' add   -P <prefix> <commit>
+'git subtree' add   -P <prefix> <repository> <commit>

Failure to specify the repository (by just specifying a local commit) fails with
the cryptic:

 warning: read-tree: emptying the index with no arguments is deprecated; use --empty
 fatal: just how do you expect me to merge 0 trees?


Furthermore, the doc paragraph for add, aside from mentionning <repository>, also
mentions a <refspec> which the synopsis does not show either.


As a sidenote it someone wants to do some maintainance, using "." as repository when
the branch to subtree-add is already locally available does not work well either
(fails with "could not find ref myremote/myhead").

-- 
Yann Dirson - Bertin Technologies