common-main: call git_setup_gettext()

common-main: call git_setup_gettext()

This should be part of every program, as otherwise users do
not get translated error messages. However, some external
commands forgot to do so (e.g., git-credential-store). This
fixes them, and eliminates the repeated code in programs
that did remember to use it.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>