filter-repo: add a script to generate a line coverage report

This also generates line coverage statistics for t/t9391/*.py, but the
point is line coverage of git-filter-repo.

Signed-off-by: Elijah Newren <newren@gmail.com>

t/run_coverage

@@ -0,0 +1,28 @@
+#!/bin/bash
+
+orig_dir=$(cd $(dirname $0) && pwd -P)
+tmpdir=$(mktemp -d)
+
+cat <<EOF >$tmpdir/.coveragerc
+[run]
+parallel=true
+data_file=$tmpdir/.coverage
+EOF
+
+cat <<EOF >$tmpdir/sitecustomize.py
+import coverage
+coverage.process_startup()
+EOF
+
+export COVERAGE_PROCESS_START=$tmpdir/.coveragerc
+export PYTHONPATH=$tmpdir:
+
+
+ls t939*.sh | xargs -n 1 bash
+
+cd $tmpdir
+python-coverage combine
+python-coverage html -d $orig_dir/report
+python-coverage report -m
+cd $orig_dir
+rm -rf $tmpdir