git JIRA hook is not idempotent

If a commit is amended after the JIRA block has been added to the tail of the commit message, the following parse error blows up the commit:

adamw:/work/civicrm/CRM/Core/CodeGen$ git commit --amend
Commited successfully.
PHP Fatal error: Uncaught exception 'Exception' with message 'Invalid JIRA key: CRM-13871:' in /work/civicrm/packages/git-footnote/src/CRM/GitFootnote/JiraFilter.php:96
Stack trace:
#0 /work/civicrm/packages/git-footnote/src/CRM/GitFootnote/JiraFilter.php(75): CRM\GitFootnote\JiraFilter->getIssue('CRM-13871:')
#1 /work/civicrm/packages/git-footnote/src/CRM/GitFootnote/AbstractWordFilter.php(13): CRM\GitFootnote\JiraFilter->filterWord(Object(CRM\GitFootnote\CommitMessage), 'CRM-13871:')
#2 /work/civicrm/packages/git-footnote/src/CRM/GitFootnote/JiraFilter.php(39): CRM\GitFootnote\AbstractWordFilter->filter(Object(CRM\GitFootnote\CommitMessage))
#3 /work/civicrm/packages/git-footnote/bin/git-footnote(18): CRM\GitFootnote\JiraFilter->filter(Object(CRM\GitFootnote\CommitMessage))
#4

thrown in /work/civicrm/packages/git-footnote/src/CRM/GitFootnote/JiraFilter.php on line 96