If a user types `set -e` in an interactive shell, and is using __git_ps1
to set
their prompt, the shell will die if the current directory isn't inside a git
repository.

This is because `set -e` instructs the shell to exit upon a command
returning a non-zero exit status, and the following command exits with
status 128:

	repo_info="$(git rev-parse --git-dir --is-inside-git-dir \
		--is-bare-repository --is-inside-work-tree \
		--short HEAD 2>/dev/null)"

I couldn't write a test to illustrate this - the test harness seems to
be preventing
`set -e` from having an effect.
	
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Signed-off-by: Tom "Ravi" Hale <tom@hale.ee>