get_oid_with_context(): match prototype and implementation

get_oid_with_context(): match prototype and implementation

The get_oid_with_context() function is declared to return an enum in
cache.h, but defined to return an int in sha1-name.c.  The compiler
notices this on AIX and rejects the build, since d1dd94b308 (Do not
print 'dangling' for cat-file in case of ambiguity - 2019-01-17) was
merged.

Return the correct type from the implementation to fix this.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>