build: fix bash completion sourcing

Sourcing functions in files from a function within a file being
sourced was giving bash a hard time. This fixes 'repo' command tab
completions.

Change-Id: Iac1b3078e20749fb474ed1270e0886cf435e24d9
Merge branch 'cm-10.2' of https://github.com/CyanogenMod/android_build into jb-mr2