git blame: ignore mass-reformatting (#1792)

Author: mr-c

.git-blame-ignore-revs

@@ -0,0 +1,5 @@
+# isort
+46e0485a889453dc178a878b5b5ebbfc7e4eb5f1
+
+# upgrade to black 20.8b1
+6fd6fdb381fe3f347627fd517a8f2dba7b0a7029

CONTRIBUTING.md

@@ -9,6 +9,9 @@ To get the code and start working on the changes you can start a console and:
 - Clone the cwltool: `git clone https://github.com/common-workflow-language/cwltool.git`
 - Switch to cwltool directory: `cd cwltool`
 
+It is suggested that you run `git config blame.ignoreRevsFile .git-blame-ignore-revs`
+to filter out mass-formatting commits from `git blame`.
+
 In order to contribute to the development of `cwltool`, the source code needs to
 pass the test before your changes are accepted. There are a couple ways to test
 the code with your changes: let `tox` manage installation and test running in