docs and completion and dashless commands

[dherring@ll.mit.edu]

First off, I'm not too keen on these dashless commands.
- We now have `man git-X` but can't run `git-X`
- A separate completion script must be installed for bash
- Lose completion for all other shells
- History expansion is complicated (e.g. `!git-push` vs `!git push`)

I tracked down "Invoke git-repo-config directly" (March 06) and related 
threads; so I somewhat understand the logic.  However the notions that 
moving bin/git-X to libexec/git-core/git-X somehow hides complexity for 
the user or fixes problems with PATH seem misguided.  Maybe there were 
some other reasons I missed?

Anyway, could someone modify INSTALL to mention 
contrib/completion/git-completion.bash and putting `git --exec-path` in 
PATH for other shells?  A man page about configuring shells for git would 
also be nice.  These docs might also instruct users to call git-config and 
set their global username and email.

Also, with the dashless commands, it may be more appropriate to distribute 
a texinfo tree than flat manpages.

Thanks,
Daniel