If you have insufficient credits to perform a full repair, you could partially repair for as much as you have.

For example, Ship A costs 10,000c to buy.
You only have 3,000c
Your ship is on 10%, and you want to repair as much as you can afford.
Click "repair to cost".
Now, assuming the repair price(to full hull) is (100% - current%)% of the original buy price, you may repair to 40%. [where current% is your ship's percentage of hull left]

Generally, this could really, only be targeted at newbs, but I'm guessing it will be a feature to-add of the devs long list ;)

/me shyly raises his hand...

... I've had moments where I needed this feature

I wouldn't want the normal repair button to do this by default. It can cost 100k+ to repair a greyhound. If I am that broke, I can probable afford a hog2 much cheaper than I can afford to repair a greyhound.

If you can't afford repairs on even the cheapest ships, it might be time to get out the freebus...