I've noticed that in the common use of English, namely in songs, there is also an extra redundant(?) "a" before a gerund, such as in a gospel song I heard
Jesus is a coming
(this particular phrase looks like quoted here).
What's the exact grammatical role of this extra inserted a? Is it the indefinite article? Why is it there? Is there any specific reason for it, or is it just bad language, slang or what? Has it developed from a pause in the speech when the speaker needed an extra time to find the word? Something like this:
Jesus is a... what the hell is he.. well, coming!
ADDED:
Based on a comment I am supposed to spell it as
Jesus is a-coming
so maybe you may comment on this, too.