util: add util/maint/show_changes_by_file.py tool

util: add util/maint/show_changes_by_file.py tool

The script diffs an upstream branch and a feature branch to find
all files which have been changed between the branches. It then
searches for all the corresponding changesets and binds them to
the files in the output.

The tool's output resembles the following:

/path/to/file/
    filename
        git-hash
        git-hash
    filename
        git-hash

/path/to/file/
    filename
        git-hash

Change-Id: Icd67953748f38dad984488c5445313eaa56fa202
Reviewed-on: https://gem5-review.googlesource.com/7861
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>