Fix rendering context lines on MR diffs

Fatih Acetrequested to merge
_acet-fix-expanding-context-lines into master

Using v-once on MR diffs was causing weird re-rendering issues. When context lines fetched, Vue wasn't updating the line numbers and line contents because of v-once

Some examples of the bugs

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/49242 and https://gitlab.com/gitlab-org/gitlab-ce/issues/49343

Edited by Fatih Acet

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