What does the apostrophe in the expression ‘I’ faith’ mean?

In older translations of Latin texts (and I would presume Greek as well), the phrase I’ faith is quite common. Examples from Plautus’ Menaechmus 2.2 and 2.3:

CYLINDRUS
I’ve catered well, and to my mind. I’ll set a good breakfast before the breakfasters. But see, I perceive Menaechmus. Woe to my back; the guests are now already walking before the door, before I’ve returned with the provisions. I’ll go and accost him. Save you, Menaechmus.
MENAECHMUS SOSICLES
The Gods bless you, whoever you are. …
CYLINDRUS
… who I am?
MESSENIO
I’ faith, not I, indeed.

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MESSENIO
I’ faith, ’tis far from surprising: courtesans have this custom; they send servant-boys and servant-girls down to the harbour; if any foreign ship comes into port, they enquire of what country it is, and what its name is; after that, at once they set themselves to work, and fasten themselves upon him; if they inveigle him, they send him home a ruined man. Now in this harbour there stands a piratical craft, against which I really think that we must be on our guard.

From the Latin, it is clear that it is an exclamation of sorts. (The first case: ‘Men.: Nōn hercle vērō.’ And the second case: ‘Mess.¹ Minimē hercle mīrum.’) The problem is not with the Latin, but with the English expression itself. Searching my usual dictionaries, I find no entries for this sort of language, which by now has become archaic. My question thus is:

• What is the apostrophe indicating? I am assuming it is a missing verb.
• What does the expression I’ faith really mean?

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¹ Apparently there is an error in either the English or Latin text here. The Latin has is as Menaechmus speaking; the English as Messeniō.