E2E Overhaul (v4)

[thockin]

This overhauls the e2e testing to:

a) Improve readability
b) Allow to run individual tests
c) Better handle ^C interruption

This is best reviewed commit-by-commit witch whitespace ignored.

Example:

$ ./test_e2e.sh -?
available tests:
    head_once_root_doesnt_exist
    head_once_root_exists_empty
    head_once_root_flag_is_weird
    head_once_root_flag_has_symlink
    non_zero_exit
    head_once_root_exists_but_is_not_git_root
    head_once_root_exists_but_fails_sanity
    default_sync_master
    head_sync
    worktree_cleanup
    readlink
    branch_sync
    branch_switch
    tag_sync
    tag_sync_annotated
    rev_sync
    rev_once
    crash_cleanup_retry
    sync_loop_timeout
    depth
    fetch_skip_depth_1
    password
    askpass_url
    exechook_success
    exechook_fail_retry
    webhook_success
    webhook_fail_retry
    webhook_fire_and_forget
    http
    submodule_sync
    submodule_sync_depth
    submodule_sync_off
    submodule_sync_shallow
    ssh
    sparse_checkout
    additional_git_configs
    export_error
    github_https

$ ./test_e2e.sh head_sync
making bin/linux_amd64/git-sync
container: e2e/git-sync:v3.2.1-134-g06aad1d__linux_amd64

<docker  build output>

test root is /tmp/git-sync-e2e.1580322924

testcase head_sync: PASS

all tests passed: cleaning up /tmp/git-sync-e2e.1580322924

This builds on and includes #446

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: thockin

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[thockin]

self-merge