If Orthodox Christians must repent of any act of violence, how does this not entail pacifism?

First, I would say that your premise:

... true repentance includes a genuine belief that facing the same situation again, one would act differently

does not accurately represent the Orthodox Christian understanding of "repentance". This notion of repentance accords with the conventional dictionary definition of feeling remorse or regret (e.g. Concise Oxford English Dictionary, 6th ed.), but it does not agree with how repentance is viewed by Orthodox.

The Greek word in Scripture for "repentance" is metanoia. Metanoia, explains Metropolitian Kallistos Ware, "means literally 'change of mind'."

In approaching God, we are to change our mind, stripping ourselves of all our habitual ways of thinking. We are to be converted not only in our will but in our intellect. We need to reverse our interior perspective, to stand the pyramid on its head.

The Orthodox Way (Kindle Locations 167-169)

As Metropolitan Kallistos further explains,

Correctly understood, repentance is not negative but positive. It means not self-pity or remorse but conversion, the re-centering of our whole life upon the Trinity. It is to look not backward with regret but forward with hope—not downwards at our own shortcomings but upwards at God's love. It is to see, not what we have failed to be, but what by divine grace we can now become; and it is to act upon what we see. To repent is to open our eyes to the light. In this sense, repentance is not just a single act, an initial step, but a continuing state, an attitude of heart and will that needs to be ceaselessly renewed up to the end of life. In the words of St Isaias of Sketis, “God requires us to go on repenting until our last breath.” “This life has been given you for repentance”, says St Isaac the Syrian. “Do not waste it on other things.”

Ibid., Kindle Locations 2053-2060

Thus, the Orthodox understanding of repentance lies not in the context of physical acts that one may or may not commit, but rather in the disposition of the heart. Military service - especially in combat - is a great spiritual burden for Orthodox Christians because it invites spiritual warfare and calls for a passionlessness that is almost impossible to attain. This is perhaps one reason that almost every single Orthodox service includes prayers for the country's armed forces.

There is, however, a great number of soldiers who are included in the Synaxaria of the Orthodox Church, including St. George the Victory-bearer, who served in the Roman Army under the emperor Diocletian, and Theodore the Recruit. Among latter day saints, we might include the recent Russian martyr Yevgeny Rodionov and the recently-reposed Archimandrite Kyrill, a decorated Red Army veteran who fought at Stalingrad.

Nonetheless, your point is taken. Basil the Great advocated for Christians to abstain from communion for a time after participating in a war: "Our fathers did not think that killing in war was murder; yet I think it advisable for such as have been guilty of it to forbear communion three years" (First Canonical Epistle, Canon XIII). A Question & Answer dialog on the Orthodox Church of America website suggests that pacifism is preferred, but perhaps impractical:

But still, if a man will be perfect and give his life totally to Christ, he will of necessity renounce military service as well as any political service which always and of necessity is involved with relativistic values and greater and lesser evils and goods. Such a man will also renounce his possessions and follow Christ totally and in everything.

Thus total pacifism is not only possible, it is the sign of greatest perfection, the perfection of the Kingdom of God. According to the Orthodox understanding, however, pacifism can never be a social or political philosophy for this world; although once again, a non-violent means to an end is always to be preferred in every case to a violent means.

When violence must be used as a lesser evil to prevent greater evils, it can never be blessed as such, it must always be repented of, and it must never be identified with perfect Christian morality.

Also, one final point of great importance is that Christians who are involved in the relativistic life of this world must resist military conscription when the state is evil. But when doing so they must not yield to anarchy, but must submit to whatever punishment is given so that their witness will be fruitful.

(This is offered as an opinion and not dogma by the OCA, but I thought I would include it as an item of interest)