Corrected `git extras update` where ${PREFIX} was not honored for `make install`

- query for $PREFIX given location of `git-extras` script
- prefix `make install` with ${PREFIX} -- empty is given if $PREFIX not
  set allowing makefile to continue to use it's default in that case.
Favoring the use of bash "Parameter Expansion" instead of "dirname" as
"dirname" has some cross-platform challenges.

- removed the superfluous stderr redirection
- switched out dirname for param expansion
Merge pull request #104 from wilmoore/bug/extras-update-to-honor-prefix

Corrected `git extras update` where ${PREFIX} was not honored for `make install`
tweak update a bit
Release 1.6.0
git-undo now keeps file changes when undoing.

Wiping out previous commits is error prone. Most people probably want to edit their
changes and commit again. Modified behavior so that when undoing a commit, changes
are left in the user directory unstaged.
Added repo age to git-summary.

Shows intitial date of first commit and how long ago that was in parentheses.
Merge pull request #107 from muhtasib/summary_repo_age

Added repo age to git-summary.
git-summary: make : line up
git-summary: shorter repo age
Release 1.7.0
Added dots for fill instead of blank spaces
Merge pull request #108 from muhtasib/git_effort_add_dots

Added dots for fill instead of blank spaces
Updating git-summary documentation, because we added a 'repo age' field to git-summary command.
Merge pull request #109 from muhtasib/docs_git_summary

Updating git-summary documentation
Added missing documenation for git-effort command
Updated git-extras to include git-effort.

Regenerated git-extras to reflect git-effort being added, and git-pull-request being removed.
Created Readme.md file on how to generate documenation.
Merge pull request #110 from muhtasib/docs_git_effort

Docs git effort
Added git-info command.

Updated Authors document to include the name of the person whose idea this was.
Added documentation for git-info command.