builtin/apply: make add_conflicted_stages_file() return -1 on error

builtin/apply: make add_conflicted_stages_file() return -1 on error

To libify `git apply` functionality we have to signal errors to the
caller instead of die()ing.

To do that in a compatible manner with the rest of the error handling
in "builtin/apply.c", add_conflicted_stages_file() should return -1
instead of calling die().

Helped-by: Eric Sunshine <redacted>
Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>