travis-ci: introduce a $jobname variable for 'ci/*' scripts

A couple of 'ci/*' scripts are shared between different build jobs: 'ci/lib-travisci.sh', being a common library, is sourced from almost every script, while 'ci/install-dependencies.sh', 'ci/run-build.sh' and 'ci/run-tests.sh' are shared between the "regular" GCC and Clang Linux and OSX build jobs, and the latter two scripts are used in the GETTEXT_POISON Linux build job as well. Our builds could benefit from these shared scripts being able to easily tell which build job they are taking part in. Now, it's already quite easy to tell apart Linux vs OSX and GCC vs Clang build jobs, but it gets trickier with all the additional Linux-based build jobs included explicitly in the build matrix. Unfortunately, Travis CI doesn't provide much help in this regard. The closest we've got is the $TRAVIS_JOB_NUMBER variable, the value of which is two dot-separated integers, where the second integer indicates a particular build job. While it would be possible to use that second number to identify the build job in our shared scripts, it doesn't seem like a good idea to rely on that: - Though the build job numbering sequence seems to be stable so far, Travis CI's documentation doesn't explicitly states that it is indeed stable and will remain so in the future. And even if it were stable, - if we were to remove or insert a build job in the middle, then the job numbers of all subsequent build jobs would change accordingly. So roll our own means of simple build job identification and introduce the $jobname environment variable in our builds, setting it in the environments of the explicitly included jobs in '.travis.yml', while constructing one in 'ci/lib-travisci.sh' as the combination of the OS and compiler name for the GCC and Clang Linux and OSX build jobs. Use $jobname instead of $TRAVIS_OS_NAME in scripts taking different actions based on the OS and build job (when installing P4 and Git LFS dependencies and including them in $PATH). The following two patches will also rely on $jobname. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>