I have an old Dell Inspiron laptop with a Core Duo processor (not Core 2 Duo) that I use as a server for Git. I believe that Raspberry Pi board is actually considerably better.
Especially since that laptop has cooling problems and often throttles down well below 1 GHz.
Of course, a direct clock speed comparison doesn't really make sense. But I would be surprised if that Pi did not run better than it.
@Zanna Anyway, I don't know how people usually use a Pi. Does one usually have a read-only root filesystem?
I've run Debian on an emulated ARM system using QEMU, and (at least as I did it) that worked pretty much just like any Debian system, with / read-write. I didn't use a boot loader (and I had no hardware acceleration for the emulator since the host was amd64 so it was very slow) but besides that it felt the same.
I don't think I've actually run Ubuntu on ARM. I think the only physical ARM device I have is my Android phone. But if I can follow in a QEMU machine, I'd be interested to.
I think you totally should use that to host a Git server because it's suitable cool and I believe quite fit for that purpose. However, I don't want to mislead you -- you don't need a separate machine to store and manage your own remote repositories...
To clarify, I'm not saying you have to or even should run Ubuntu. I don't know what OS you should use. I don't think most Raspberry Pi users use Ubuntu.
@Zanna I expect the hard part to be setting up the operating system.
I don't expect that to be hard.
Just harder (or anyway more time consuming) than setting it up to use as a Git server.