issue-4: Make get-git compatible with git download site

  * Update dl-git based on new git download site structure
  * Add new functions borrowed from gnulib's bootstrap
  * Update dl-git and get-git to use new functions
  * Have get-git install git-prompt.sh and git-completion.bash

Signed-off-by: Mike Detwiler <mike@mikedetwiler.com>
Merge pull request #5 from detwiler/issue-4

issue-4: Make get-git compatible with git download site