Being able to commit parts of diffs can bite you if you aren't careful.

Fixes Homebrew/homebrew#136

The problem was that git checkout -b doesn't work the second time. This simple solution works by never creating a local branch.

GitDownloadStrategy and MercurialDownloadStrategy
now can be used like this:

  head 'git://server/repo.git', :branch => 'stable'
  head 'hg://server/repo/', :tag => '1.0.4'

Because people didn't realise it was actually curl and thus supports eg. ftp too.

Closes Homebrew/homebrew#58

Otherwise you run the risk of not running the exact version / make of the utility you planned.

Fixes Homebrew/homebrew#48

Really we need to do this formula too, so I guess a make and cmake function are on the way…

Don't delete it if it doesn't exist.

Only delete the file that is downloaded if an error occurs while downloading it.

Otherwise next time it will assume the half-file is the whole file and fail
the md5 check.

I removed the rename and mv functions as when I wrote the tests I realised the function implied the pathname object would be updated to reflect the moved or renamed file. However that cannot be done. Also frankly I think writing it out in full makes clearer code.

I confirmed this change with all relevant contributors first.

Because formula don't get named unless the brew kit instantiates them accessory formula were getting named "__UNKNOWN__". Which sucks.

This isn't ideal for me as I made the naming use @name and @version to ensure unique naming. Now it is possible to have name clashes in the cache. So I need to solve it better at some point.

Fix git download strategy