sha1_file: introduce a constant for max header length

sha1_file: introduce a constant for max header length

There were several instances of 32 sprinkled throughout this file, all
of which were used for allocating a buffer to store the header of an
object.  Introduce a constant, MAX_HEADER_LEN, for this purpose.

Note that this constant is slightly larger than required; the longest
possible header is 28 (7 for "commit", 1 for a space, 20 for a 63-bit
length in decimal, and 1 for the NUL).  However, the overallocation
should not cause any problems, so leave it as it is.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>