Update git.mk
Misc warning and other fixes
Revert "Demo reads text from file"

This reverts commit 9f4bb3d08af39d9e11dd6dc2ab2fe0a3847c7191.

This doesn't work for out-of-tree builds.  Users can always
provide text from a file from the commandline:

./glyphy-demo font.ttf "$(<test.txt)"