Imperative form is recommended in Git documentation itself and it's what the Linux kernel uses[1].

It makes a lot of sense if you don't view DVCSes as a fancy history mechanism but as a patch management system. Here, take this bunch of patches. What will I do if I'll merge them? I'll fix X, add option Z and port Q to ARM. Now let's say I also want to backport some patches to a stable branch, I'll take one from this branch, some from others and I'll fix X, fix Y and fix Z, but on ARM nothing works without the ARM commit, so on that branch I'll port Q to ARM.

[1] http://git.kernel.org/?p=git/git.git;a=blob;f=Documentation/...