Tolerate zlib deflation with window size < 32Kb

JGit currently identifies loose objects as 'corrupt' if they've been
deflated using a window size less than 32Kb, because the
isStandardFormat() function doesn't recognise the header
byte as a zlib header. This patch makes the method tolerant of
all valid window sizes (15-bit to 8-bit) - but doesn't sacrifice
it's accuracy in distingushing the standard loose-object format
from the experimental (now abandoned) format. It's based on a patch
which has been merged into C-Git master branch:

https://github.com/git/git/commit/7f684a2aff636f44a506

On memory constrained systems zlib may use a much smaller window
size - working on Agit, I found that Android uses a 4KB window;
giving a header byte of 0x48, not 0x78. Consequently all loose
objects generated by the Android platform appear 'corrupt' :(

It might appear that this patch changes isStandardFormat() to the
point where it could incorrectly identify the experimental format as
the standard one, but the two criteria (bitmask & checksum) can only
give a false result for an experimental object where both of the
following are true:

1) object size is exactly 8 bytes when uncompressed (bitmask)
2) [single-byte in-pack git type&size header] * 256
   + [1st byte of the following zlib header] % 31 = 0 (checksum)

As it happens, for all possible combinations of valid object type
(1-4) and window bits (0-7), the only time when the checksum will be
divisible by 31 is for 0x1838 - ie object type *1*, a Commit - which,
due the fields all Commit objects must contain, could never be as
small as 8 bytes in size.

Given this, the combination of the two criteria (bitmask & checksum)
always correctly determines the buffer format, and is more tolerant
than the previous version.

References:

Android uses a 4KB window for deflation:
http://android.git.kernel.org/?p=platform/libcore.git;a=blob;f=luni/src/main/native/java_util_zip_Deflater.cpp;h=c0b2feff196e63a7b85d97cf9ae5bb2583409c28;hb=refs/heads/gingerbread#l53

Code snippet searching for false positives with the zlib checksum:
https://gist.github.com/1118177

Change-Id: Ifd84cd2bd6b46f087c9984fb4cbd8309f483dec0

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/UnpackedObject.java

@@ -270,14 +270,35 @@ public class UnpackedObject {
 	}
 
 	private static boolean isStandardFormat(final byte[] hdr) {
-		// Try to determine if this is a standard format loose object or
-		// a pack style loose object. The standard format is completely
-		// compressed with zlib so the first byte must be 0x78 (15-bit
-		// window size, deflated) and the first 16 bit word must be
-		// evenly divisible by 31. Otherwise its a pack style object.
-		//
+		/*
+		 * We must determine if the buffer contains the standard
+		 * zlib-deflated stream or the experimental format based
+		 * on the in-pack object format. Compare the header byte
+		 * for each format:
+		 *
+		 * RFC1950 zlib w/ deflate : 0www1000 : 0 <= www <= 7
+		 * Experimental pack-based : Stttssss : ttt = 1,2,3,4
+		 *
+		 * If bit 7 is clear and bits 0-3 equal 8, the buffer MUST be
+		 * in standard loose-object format, UNLESS it is a Git-pack
+		 * format object *exactly* 8 bytes in size when inflated.
+		 *
+		 * However, RFC1950 also specifies that the 1st 16-bit word
+		 * must be divisible by 31 - this checksum tells us our buffer
+		 * is in the standard format, giving a false positive only if
+		 * the 1st word of the Git-pack format object happens to be
+		 * divisible by 31, ie:
+		 *      ((byte0 * 256) + byte1) % 31 = 0
+		 *   =>        0ttt10000www1000 % 31 = 0
+		 *
+		 * As it happens, this case can only arise for www=3 & ttt=1
+		 * - ie, a Commit object, which would have to be 8 bytes in
+		 * size. As no Commit can be that small, we find that the
+		 * combination of these two criteria (bitmask & checksum)
+		 * can always correctly determine the buffer format.
+		 */
 		final int fb = hdr[0] & 0xff;
-		return fb == 0x78 && (((fb << 8) | hdr[1] & 0xff) % 31) == 0;
+		return (fb & 0x8f) == 0x08 && (((fb << 8) | hdr[1] & 0xff) % 31) == 0;
 	}
 
 	private static InputStream inflate(final InputStream in, final long size,