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Git 2.13.0 segfaults on Solaris SPARC due to DC_SHA1=YesPlease being on by default

Since 2.13.0 just running "git status" on a newly init'd repo on
Solaris SPARC[1] segfaults. Michael (CC'd) reported this issue on #git
and I helped him debug it.

Just compiling with BLK_SHA1=YesPlease solves the issue.

There are at least two different issues with DC_SHA1 here:

 * We don't properly detect that this platform is big endian. The
check at the top of sha1dc/sha1.c needs to test for _BIG_ENDIAN. This
comes from sys/isa_defs.h which (I'm told by #solaris) is included on
Solaris by default, at least by stdio.h.

Hacking the endian detection makes t0013-sha1dc.sh pass.

  * Even with that & the test passing just a plain "git init x && cd x
&& touch A && git add A && git commit" will segfault.

This is some bug in the sha1dc code, presumably some big endian issue
that's not resolved by the change above. Backtrace for that (censored
actual author info):

 Program received signal SIGSEGV, Segmentation fault.
 [Switching to Thread 1 (LWP 1)]
 0x002f8c84 in sha1_compression_states (ihv=0xffbf8268, m=0x398ad5,
W=0xffbf82fc, states=0xffbf857c) at sha1dc/sha1.c:291
 291             SHA1COMPRESS_FULL_ROUND1_STEP_LOAD(a, b, c, d, e, m,
W, 0, temp);
 (gdb) bt
 #0  0x002f8c84 in sha1_compression_states (ihv=0xffbf8268,
m=0x398ad5, W=0xffbf82fc, states=0xffbf857c) at sha1dc/sha1.c:291
 #1  0x00300b60 in sha1_process (ctx=0xffbf8260, block=0x398ad5) at
 #2  0x00301188 in SHA1DCUpdate (ctx=0xffbf8260,
     buf=0x398ad5 "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef\nauthor Au
Thor <au.thor@xxxxxxxxxxx> 123456789 +0000\ncommitter Au Thor
<au.thor@xxxxxxxxxxx> 123456789
                  +0000\n\nBlah Blah"..., len=220)
     at sha1dc/sha1.c:1731
 #3  0x0030168c in git_SHA1DCUpdate (ctx=0xffbf8260, vdata=0x398aa0,
len=273) at sha1dc/sha1.c:1808
 #4  0x002a6f7c in write_sha1_file_prepare (buf=0x398aa0, len=273,
type=0x959c8 "commit", sha1=0xffbfd630 "",
     hdr=0xffbf8c28 "commit 273", hdrlen=0xffbf8c24) at sha1_file.c:3207
 #5  0x002a71ac in hash_sha1_file (buf=0x398aa0, len=273, type=0x959c8
"commit", sha1=0xffbfd630 "") at sha1_file.c:3266
 #6  0x002a25f8 in check_sha1_signature (sha1=0xffbfdbb8
"\375\067\356\337\002", map=0x398aa0, size=273, type=0x959c8 "commit")
     at sha1_file.c:1644
 #7  0x0022816c in parse_object (sha1=0xffbfdbb8
"\375\067\356\337\002") at object.c:269
 #8  0x0027c258 in get_reference (revs=0xffbfdc88, name=0xa87f0
"HEAD", sha1=0xffbfdbb8 "\375\067\356\337\002", flags=0)
     at revision.c:196
 #9  0x00284714 in setup_revisions (argc=0, argv=0x0, revs=0xffbfdc88,
opt=0xffbfdc74) at revision.c:2295
 #10 0x002ee4d8 in wt_status_collect_changes_index (s=0x348ea8
<s.24114>) at wt-status.c:585
 #11 0x002eeae8 in wt_status_collect (s=0x348ea8 <s.24114>) at wt-status.c:701
 #12 0x000db4fc in cmd_status (argc=0, argv=0xffbfe6fc, prefix=0x0) at
 #13 0x000acc24 in run_builtin (p=0x340ccc <commands+1200>, argc=1,
argv=0xffbfe6fc) at git.c:371
 #14 0x000acf98 in handle_builtin (argc=1, argv=0xffbfe6fc) at git.c:572
 #15 0x000ad1a4 in run_argv (argcp=0xffbfe66c, argv=0xffbfe670) at git.c:624
 #16 0x000ad3fc in cmd_main (argc=1, argv=0xffbfe6fc) at git.c:701
 #17 0x001747a0 in main (argc=5, argv=0xffbfe6ec) at common-main.c:43

1. Relevant part of uname -a: "SunOS <hostname> 5.11 11.3 sun4v sparc sun4v".