Move config variables into an ignored file

.gitignore

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+config.bash

config.bash.example

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+# Configuration for the repo tools, to be evaluated when they execute.
+#
+# For shell config (e.g., to be sourced into .zshrc) see init.bash
+
+REPOSITORY_BASE=~/src
+
+# If a variable is only to be used by a single script, by custom it will be
+# prefixed with the name of the script.
+
+REPO_DEFAULT_SEARCH_PREFIX="github.com/buildkite"