Makefile: BLK_SHA1 does not require fast htonl() and unaligned loads

block-sha1/ is fast on most known platforms.  Clarify the Makefile to
be less misleading about that.

Early versions of block-sha1/ explicitly relied on fast htonl() and
fast 32-bit loads with arbitrary alignment.  Now it uses those on some
arches but the default behavior is byte-at-a-time access for the sake
of arches like ARM, Alpha, and their kin and it is still pretty fast
on these arches (fast enough to supersede the mozilla SHA1
implementation and the hand-written ARM assembler implementation that
were bundled before).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -84,9 +84,8 @@ all::
 # specify your own (or DarwinPort's) include directories and
 # library directories by defining CFLAGS and LDFLAGS appropriately.
 #
-# Define BLK_SHA1 environment variable if you want the C version
-# of the SHA1 that assumes you can do unaligned 32-bit loads and
-# have a fast htonl() function.
+# Define BLK_SHA1 environment variable to make use of the bundled
+# optimized C SHA1 routine.
 #
 # Define PPC_SHA1 environment variable when running make to make use of
 # a bundled SHA1 routine optimized for PowerPC.