Merge branch 'ab/mailmap'

Merge branch 'ab/mailmap'

Clean-up docs, codepaths and tests around mailmap.

* ab/mailmap: (22 commits)
  shortlog: remove unused(?) "repo-abbrev" feature
  mailmap doc + tests: document and test for case-insensitivity
  mailmap tests: add tests for empty "<>" syntax
  mailmap tests: add tests for whitespace syntax
  mailmap tests: add a test for comment syntax
  mailmap doc + tests: add better examples & test them
  tests: refactor a few tests to use "test_commit --append"
  test-lib functions: add an --append option to test_commit
  test-lib functions: add --author support to test_commit
  test-lib functions: document arguments to test_commit
  test-lib functions: expand "test_commit" comment template
  mailmap: test for silent exiting on missing file/blob
  mailmap tests: get rid of overly complex blame fuzzing
  mailmap tests: add a test for "not a blob" error
  mailmap tests: remove redundant entry in test
  mailmap tests: improve --stdin tests
  mailmap tests: modernize syntax & test idioms
  mailmap tests: use our preferred whitespace syntax
  mailmap doc: start by mentioning the comment syntax
  check-mailmap doc: note config options
  ...