[gitlab-ci] Increase git depth.
[gitlab-ci] Increase git depth.

To avoid massive failures in second stage of CI build when a new PR has been merged
in master since then. Example: https://gitlab.com/coq/coq/pipelines/38528858.

ppedrot/coq,maximedenes/coq,SkySkimmer/coq,ppedrot/coq,coq/coq,ejgallego/coq,coq/coq,ppedrot/coq,silene/coq,SkySkimmer/coq,silene/coq,ejgallego/coq,JasonGross/coq,Zimmi48/coq,ejgallego/coq,maximedenes/coq,JasonGross/coq,silene/coq,ppedrot/coq,ejgallego/coq,JasonGross/coq,gares/coq,coq/coq,JasonGross/coq,maximedenes/coq,maximedenes/coq,Zimmi48/coq,gares/coq,Matafou/coq,coq/coq,Matafou/coq,gares/coq,maximedenes/coq,Matafou/coq,gares/coq,gares/coq,SkySkimmer/coq,Zimmi48/coq,Zimmi48/coq,SkySkimmer/coq,ppedrot/coq