travis-ci: don't build Git for the static analysis job

The static analysis job on Travis CI builds Git ever since it was
introduced in d8245bb3f (travis-ci: add static analysis build job to
run coccicheck, 2017-04-11).  However, Coccinelle, the only static
analysis tool in use, only needs Git's source code to work and it
doesn't care about built Git binaries at all.

Spare some of Travis CI's resources and don't build Git for the static
analysis job unnecessarily.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.travis.yml

@@ -71,7 +71,7 @@ matrix:
           packages:
           - coccinelle
       before_install:
-      # "before_script" that builds Git is inherited from base job
+      before_script:
       script: ci/run-static-analysis.sh
       after_failure:
     - env: Documentation