[vim] Detect git-cl description files as gitcommit files.

This way when vim is invoked to edit CL descriptions by `git cl upload`,
and `git cl description`, it will automatically configure `textwidth`,
and line highlighting.

BUG=none

Review-Url: https://codereview.chromium.org/2101703002
Cr-Commit-Position: refs/heads/master@{#402309}
1 file changed