added code to set GIT_EXEC_PATH and the tests

[tiff-ritz]

This is in response to Issue 137 in the heroku-buildpack-apt repo.

If git is in the project.toml file - make sure GIT_EXEC_PATH is correctly defined after successful installation or if it is already installed.

Added unit tests

[tiff-ritz]

Adding each env var to the code is not a feasable approach long term. Kept this work in a branch in my fork

Closing this PR and am going to open a new one with my configurable environment variable solution.

[edmorley]

Adding each env var to the code is not a feasable approach long term

So whilst normally I would agree, I think we do need to add handling in this CNB for commonly used packages (by setting specific env vars), since otherwise it leads to poor UX if such packages don't work out of the box.

[tiff-ritz]

Adding each env var to the code is not a feasable approach long term

So whilst normally I would agree, I think we do need to add handling in this CNB for commonly used packages (by setting specific env vars), since otherwise it leads to poor UX if such packages don't work out of the box.

In another PR, I extended the project.toml file:

[_]
schema-version = "0.2"

[com.heroku.buildpacks.deb-packages]
install = [
    { name = "git", 
      env = { "GIT_EXEC_PATH" = "{install_dir}/usr/lib/git-core", 
              "GIT_TEMPLATE_DIR" = "{install_dir}/usr/share/git-core/templates" }
    },
    { name = "babeld" },
    { name = "ghostscript",
      skip_dependencies = true,
      force = true,
      env = { "GS_LIB" = "{install_dir}/var/lib/ghostscript" }
    },
]

so these can be defined per package and added after the installation.

I understand what you are saying about working out of the box.