Ignore commits with the fixup! autosquash flag

Because:

* `git-commit` uses a `fixup!` prefix as a flag for autosquash with
  interactive rebase.
* The interactive rebase will set the commit to fixup, which will
  discard the commit message, so there's no need to validate if the flag
  is present at the start of the subject.

This change:

* Checks for the `fixup!` prefix in the subject and breaks out of the
  validation if present.
* Adds a test to verify the validation is skipped.

Author: tommarshall

hook.sh

@@ -115,7 +115,10 @@ validate_commit_message() {
 
   # if the commit is empty there's nothing to validate, we can return here
   COMMIT_MSG_STR="${COMMIT_MSG_LINES[*]}"
-  test -n "${COMMIT_MSG_STR[*]// }" || return;
+  test -z "${COMMIT_MSG_STR[*]// }" && return;
+
+  # if the commit subject starts with 'fixup! ' there's nothing to validate, we can return here
+  [[ $COMMIT_SUBJECT == 'fixup! '* ]] && return;
 
   # 1. Separate subject from body with a blank line
   # ------------------------------------------------------------------------------

test/validation.bats

@@ -24,6 +24,17 @@ EOF
   assert_success
 }
 
+@test "validation: ignores commits with the fixup! autosquash flag" {
+  echo "n" > $FAKE_TTY
+  run git commit -m "$(cat <<EOF
+fixup! Add foo bar string to my_file - As requested by Jon
+Another line in the body that runs to longer than 72 characters in length
+EOF
+)"
+
+  assert_success
+}
+
 # 0. Good commits - control
 # ------------------------------------------------------------------------------