jit-diff: automatically discover configuration

This addition would allow for automatic discovery of a typical configuration for diff'ing. When jit-diff is run in this mode it builds up a context and fills out the config file. At the high level this looks in some default locations, finds the current branch and fork-point, downloads the baseline build via 'cijobs', and writes out the config file to track this point. As of this writing I think that all the features we need are currently in the tool and just need to be wrapped up together.

Steps:
- [ ] Look for default locations for coreclr adjacent to the jitutils folder.
- [ ] Set up default output directory
- [ ] Use "git name-rev HEAD" or the like to discover the branch that the repo is on.
- [ ] Use "git merge-base $feature master" to discover the fork point from the mainline
- [ ] Download build associated with the fork-point via cijobs from the ci and install it in the tools area of the output directory.
- [ ] Set all defaults into the config file.
- [ ] Run diff.