Your project should almost always use the past tense. In any case, the project should always use the same tense for consistency and clarity.
I understand some of the other arguments arguing to use the present tense, but they usually don't apply. The following bullet points are common arguments for writing in the present tense, and my response.
This is the most correct reason one would want to use the present tense, but only with the right style of project. This manner of thinking considers all commits as optional improvements or features, and you are free to decide which commits to keep and which to reject in your particular repository.
This argument works if you are dealing with a truly distributed project. If you are dealing with a distributed project, you are probably working on an open source project. And it is probably a very large project if it is really distributed. In fact, it's probably either the Linux kernel or Git. Since Linux is likely what caused Git to spread and gain in popularity, it's easy to understand why people would consider its style the authority. Yes, the style makes sense with those two projects. Or, in general, it works with large, open source, distributed projects.
That being said, most projects in source control do not work like this. It is usually incorrect for most repositories. It's a modern way of thinking about a commits: Subversion (SVN) and CVS repositories could barely support this style of repository check-ins. Usually an integration branch handled filtering bad check-ins, but those generally weren't considered "optional" or "nice-to-have features".
In most scenarios, when you are making commits to a source repository, you are writing a journal entry which describes what changed with this update, to make it easier for others in the future to understand why a change was made. It generally isn't an optional change - other people in the project are required to either merge or rebase on it. You don't write a diary entry such as "Dear diary, today I meet a boy and he says hello to me.", but instead you write "I met a boy and he said hello to me."
Finally, for such non-distributed projects, 99.99% of the time a person will be reading a commit message is for reading history - history is read in the past tense. 0.01% of the time it will be deciding whether or not they should apply this commit or integrate it into their branch/repository.
No, I guarantee you that the majority of projects ever logged in a version control system have had their history in the past tense (I don't have references, but it's probably right, considering the present tense argument is new since Git). "Revision" messages or commit messages in the present tense only started making sense in truly distributed projects - see the first point above.
See the first point. 99.99% of the time a person will be reading a commit message is for reading history - history is read in the past tense. 0.01% of the time it will be deciding whether or not they should apply this commit or integrate it into their branch/repository. 99.99% beats 0.01%.
I've never seen a good argument that says use improper tense/grammar because it's shorter. You'll probably only save 3 characters on average for a standard 50 character message. That being said, the present tense on average will probably be a few characters shorter.
Tickets are written as either something that is currently happening (e.g. the app is showing the wrong behavior when I click this button), or something that needs to be done in the future (e.g. the text will need a review by the editor).
History (i.e. commit messages) is written as something that was done in the past (e.g. the problem was fixed).