Since presumably (1) you're checking in something that you did, and (2) the change has already happened (otherwise you'd have nothing to check in), I'd go with the simple past:

Changed function argument type to int.

If you're checking in a change on behalf of a team, I could also see using:

Function argument type changed to int.

The simple past is the least wordy tense — no helping verbs, no "now"— and is generally the easiest to understand.

I wouldn't suggest using a continuous present or future tense such as "changing", because then the log becomes unclear about what was changed when. _{^{(Akin to the problem of distances in driving directions: when it says "left turn — 5 miles", does that mean the left turn is 5 miles ahead, or do I make a left turn now and then go 5 miles until the next step?)}}

“Guys” can be used in English as gender neutral to refer to a group of mixed gender.

You will even hear women refer to other women as “guys.”

The closest linguistic equivalent with a feminine tilt would be “gals.” “Guys and gals” is a rather informal variant of “ladies and gentlemen.” (Note the reverse order.)

Edit: As noted by @kitukwfyer in the question comments, it is possible for “gals” to be derogatory when used by itself so be careful. It’s not likely to be a problem used in the pairing above. Used carefully it’s probably safe, but used by itself it could be heard as mockery.

Saying the following is unlikely to offend anyone:

“The gals talked in the kitchen while the guys went out to see Fred’s new truck.”

But walking up to a couple women on a street corner and saying this might get a strange reaction:

“Excuse me gals, where is the grocery store?”

Saying this to a couple girls might make them doubt you:

“Hey gals come with me.”

But if you are explaining some game instructions and say the following it’s likely to be fine:

“Alright, guys on this side of the room, gals on the other.”