[WIP] Add git-pull-all command

[nicolaiskogheim]

Track working directories once, and then use git pull-all to pull each repository in one go later.

This is just #77 rebased onto master. The only edits I did was remove trailing whitespace.

I don't think I would use this command, but a couple of guys showed interest in the comments.
If anyone has thoughts on whether git pull-all should be included or not, please share.

I haven't reviewed or tested the code, hence the WIP label, but I will after we've decided to take it in. If we do, that is.

[qw3rtman]

Just to clarify, does this pull all branches from remote?

@hughsk

[nicolaiskogheim]

does this pull all branches from remote?

No.

There are two cases:

1. if $# -eq 0 then git pull in each repository
2. else git pull "$@", which means you can give arguments, but passing in branch names wouldn't make sense, except when you want to only pull the master branch in every repo.

[qw3rtman]

My bad, I should have looked at the man page in this PR.

This looks like a useful feature to me (especially for those who keep all their repositories in a central directory).