It is somewhat surprising when git-cl, which acts as a git subcommand, launches
a different editor. In particular, git has a config option (core.editor) which
specifies the editor that should be used. Since we already respect $GIT_EDITOR,
it makes sense for git-cl to respect core.editor and $VISUAL as well.

R=maruel@chromium.org
BUG=237504

Review URL: https://chromiumcodereview.appspot.com/14854003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@198101 0039d316-1c4b-4281-b951-d872f2087c98