misc: updated shabang for python script

The default python on MacOS doesn’t have an alias to python2.
The official python version supported in gem5 is Python2.7.

This patch updates the shabang according to the version required in gem5.

Change-Id: I9533c0f7858b5b3cab0ef101be1ee5cd718105b0
Reviewed-on: https://gem5-review.googlesource.com/c/15375
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

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>