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

builtin/apply: make parse_single_patch() 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, parse_single_patch() should return a negative
integer instead of calling die().

Let's do that by using error() and let's adjust the related test
cases accordingly.

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