The order was disbanded in 1312 which is IIRC the same year the movie uses. As for the 'longsword' you have to factor in that weapons of all kinds appear earlier and later than their most common 'period' of usage. That was one of the challenges I faced when writing the weapon rules for Brink of Battle. So, its not necessarily anachronistic for him to have one in 1312.
And now I have to go wash my mouth out with soap cuz you said Kingdom of Heaven...
The film is (very roughly) based on events surrounding the French invasion of 1216 - so about 100 years earlier.
Agreed on common usage, but is there a single surviving example or an illustration of a long sword in NW Europe (or even in the Holy Land) at this time or even within one or two generations?