git-prompt: make __git_eread intended use explicit

git-prompt: make __git_eread intended use explicit

__git_eread is used to read a single line of a given file (if it exists)
into a single variable stripping the EOL.
This patch removes the unused capability to split file contents into tokens
by passing multiple variable names. Add a comment and explicitly use $2
instead of misleading $@ as argument to the read builtin command.

Signed-off-by: Robert Abel <redacted>
Signed-off-by: Junio C Hamano <redacted>