Given a simple repository
$ touch empty ; git add empty ; git commit -m 'My commit' [master (root-commit) e9cc922] My commit 1 file changed, 0 insertions(+), 0 deletions(-) create mode 100644 empty $ git tag -a -m 'My tag' mytag
we notice that one of these things is not like the others, emphasis added below. Why do tree objects encode SHA-1 hashes directly as sequences of twenty unsigned bytes whereas commits and tags encode them as forty ASCII hex digits?
I noticed the discrepancy while investigating the answer to a related question about git internal storage.
$ for f in .git/objects/??/* ; do echo $f ; openssl zlib -d < "$f" | xxd ; echo ; done .git/objects/41/7c01c8795a35b8e835113a85a5c0c1c77f67fb 00000000: 7472 6565 2033 3300 3130 3036 3434 2065 tree 33.100644 e 00000010: 6d70 7479 00e6 9de2 9bb2 d1d6 434b 8b29 mpty........CK.) 00000020: ae77 5ad8 c2e4 8c53 91 .wZ....S. .git/objects/da/867754dac38858c708292f49de41b7eff6f5d7 00000000: 7461 6720 3133 3400 6f62 6a65 6374 2065 tag 134.object e 00000010: 3963 6339 3232 3632 6561 3734 3138 3864 9cc92262ea74188d 00000020: 6530 3436 6563 6166 3265 3363 3063 3061 e046ecaf2e3c0c0a 00000030: 3436 3966 3734 320a 7479 7065 2063 6f6d 469f742.type com 00000040: 6d69 740a 7461 6720 6d79 7461 670a 7461 mit.tag mytag.ta 00000050: 6767 6572 2047 7265 6720 4261 636f 6e20 gger Greg Bacon 00000060: 3c67 6261 636f 6e40 666f 6f2e 696e 7661 <gbacon@foo.inva 00000070: 6c69 643e 2031 3435 3737 3035 3338 3820 lid> 1457705388 00000080: 2d30 3630 300a 0a4d 7920 7461 670a -0600..My tag. .git/objects/e6/9de29bb2d1d6434b8b29ae775ad8c2e48c5391 00000000: 626c 6f62 2030 00 blob 0. .git/objects/e9/cc92262ea74188de046ecaf2e3c0c0a469f742 00000000: 636f 6d6d 6974 2031 3732 0074 7265 6520 commit 172.tree 00000010: 3431 3763 3031 6338 3739 3561 3335 6238 417c01c8795a35b8 00000020: 6538 3335 3131 3361 3835 6135 6330 6331 e835113a85a5c0c1 00000030: 6337 3766 3637 6662 0a61 7574 686f 7220 c77f67fb.author 00000040: 4772 6567 2042 6163 6f6e 203c 6762 6163 Greg Bacon <gbac 00000050: 6f6e 4066 6f6f 2e69 6e76 616c 6964 3e20 on@foo.invalid> 00000060: 3134 3537 3730 3531 3736 202d 3036 3030 1457705176 -0600 00000070: 0a63 6f6d 6d69 7474 6572 2047 7265 6720 .committer Greg 00000080: 4261 636f 6e20 3c67 6261 636f 6e40 666f Bacon <gbacon@fo 00000090: 6f2e 696e 7661 6c69 643e 2031 3435 3737 o.invalid> 14577 000000a0: 3035 3137 3620 2d30 3630 300a 0a4d 7920 05176 -0600..My 000000b0: 636f 6d6d 6974 0a commit.