architecture sanity: rename benchmark_result.commit_repo_url to benchmark_result.repo_url

[austin3dickey]

In #1453 I introduced the benchmark_result.commit_repo_url column. @jgehrcke has pointed out that the name doesn't quite make sense (naming is hard!):

conbench/conbench/entities/benchmark_result.py

Lines 87 to 90 in cf7931f

	# Note(JP): maybe it is easier to think of this as just "repo_url" because
	# while it is not required that each result is associated with a particular
	# commit, but instead it is required to be associated with a (one!) code
	# repository as identified by its user-given repository URL.

I had originally prefixed this column name with commit_ thinking it was a good namespace for other future columns that had to do with git information. In this project in the past we've sometimes used "github", "commit", and "git" interchangeably, as a tag that a certain object has something to do with git information, and this really doesn't make any sense. (I find this pretty funny.)

A better namespace in this case would be git_. But really, a namespace is probably not required. We have no current plans to put more git information onto the benchmark_result table, because the commit table remains useful. So I agree with @jgehrcke that we should rename this column to repo_url, in order to lower the barrier to entry for future developers, and remove confusion from the codebase.

This will require a DB migration, but renaming columns is extremely fast in modern Postgres versions.