travis-ci: express Linux/OS X dependency versions more clearly

The Git Travis CI OSX build always installs the latest versions of Git LFS and
Perforce via brew and the Linux build installs fixed versions. Consequently new
LFS/Perforce versions can break the OS X build even if there is no change in
Git.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.travis.yml

@@ -22,8 +22,11 @@ addons:
 env:
   global:
     - DEVELOPER=1
-    - P4_VERSION="16.1"
-    - GIT_LFS_VERSION="1.2.0"
+    # The Linux build installs the defined dependency versions below.
+    # The OS X build installs the latest available versions. Keep that
+    # in mind when you encounter a broken OS X build!
+    - LINUX_P4_VERSION="16.1"
+    - LINUX_GIT_LFS_VERSION="1.2.0"
     - DEFAULT_TEST_TARGET=prove
     - GIT_PROVE_OPTS="--timer --jobs 3 --state=failed,slow,save"
     - GIT_TEST_OPTS="--verbose --tee"
@@ -38,17 +41,17 @@ before_install:
     linux)
       mkdir --parents custom/p4
       pushd custom/p4
-        wget --quiet http://filehost.perforce.com/perforce/r$P4_VERSION/bin.linux26x86_64/p4d
-        wget --quiet http://filehost.perforce.com/perforce/r$P4_VERSION/bin.linux26x86_64/p4
+        wget --quiet http://filehost.perforce.com/perforce/r$LINUX_P4_VERSION/bin.linux26x86_64/p4d
+        wget --quiet http://filehost.perforce.com/perforce/r$LINUX_P4_VERSION/bin.linux26x86_64/p4
         chmod u+x p4d
         chmod u+x p4
         export PATH="$(pwd):$PATH"
       popd
       mkdir --parents custom/git-lfs
       pushd custom/git-lfs
-        wget --quiet https://github.com/github/git-lfs/releases/download/v$GIT_LFS_VERSION/git-lfs-linux-amd64-$GIT_LFS_VERSION.tar.gz
-        tar --extract --gunzip --file "git-lfs-linux-amd64-$GIT_LFS_VERSION.tar.gz"
-        cp git-lfs-$GIT_LFS_VERSION/git-lfs .
+        wget --quiet https://github.com/github/git-lfs/releases/download/v$LINUX_GIT_LFS_VERSION/git-lfs-linux-amd64-$LINUX_GIT_LFS_VERSION.tar.gz
+        tar --extract --gunzip --file "git-lfs-linux-amd64-$LINUX_GIT_LFS_VERSION.tar.gz"
+        cp git-lfs-$LINUX_GIT_LFS_VERSION/git-lfs .
         export PATH="$(pwd):$PATH"
       popd
       ;;