Change constant `git_ignore_lines` element `.Rdata` to`.RData`

[dchiu911]

I believe the standard R workspace is called .RData. However, the constant git_ignore_lines has the value ".Rdata". So if the user already has .RData in their .gitignore and runs usethis::use_git() to initialize a repo, they'd have .Rdata added when .RData is the same

[jennybc]

Yeah from the history it looks like use_git() used to ignore .Rhistory, .RData, and .Rproj.user and git_vaccinate() ignored .Rproj.user, .Rhistory, .Rdata, .httr-oauth, and .DS_Store and then they were harmonized (https://github.com/r-lib/usethis/commit/26d2c4460ab9cb605a3d0d6682cfcf1f20c8285b) with .Rdata surviving, when it seems like .RData would make more sense.

Would you like to make a PR? This should have a NEWS bullet.