Add support for per-repository bugtraq configuration

Imported the reference implementation contributed by syntevo which
is used in their SmartGit product.  You may create a bugtraq config
section inf your .git/config file OR you may add a .gitbugtraq file
to the root of your repository.

Example:

    [bugtraq "issues"]
        url = http://code.google.com/p/gitblit/issues/detail?id=%BUGID%
        logRegex = "[Ii]ssue[-#:\\s]{1}\\d+"
        logRegex1 = "\\d+"
    [bugtraq "[pullrequests"]
        url = "https://github.com/gitblit/gitblit/pull/%BUGID%"
        logRegex = "(?:pull request|pull|pr)\\s*[-#]?([0-9]+)"

Change-Id: Iaba305bf4280d08cc4d1abf533c2f1365470a43f