It is also possible to decompress it using standard shell-script + gzip, if you don't have, or want to use openssl or other tools.
The trick is to prepend the gzip magic number and compress method to the actual data from zlib.compress:

printf "\x1f\x8b\x08\x00\x00\x00\x00\x00" |cat - /tmp/data |gzip -dc >/tmp/out

Edits:
@d0sboots commented: For RAW Deflate data, you need to add 2 more null bytes:
→ "\x1f\x8b\x08\x00\x00\x00\x00\x00\x00\x00"

This Q on SO gives more information about this approach. An answer there suggests that there is also an 8 byte footer.

Users @Vitali-Kushner and @mark-bessey reported success even with truncated files, so a gzip footer does not seem strictly required.

@tobias-kienzler suggested this function for the bashrc:
zlibd() (printf "\x1f\x8b\x08\x00\x00\x00\x00\x00" | cat - "$@" | gzip -dc)

Looks like zlib compressed data, here's more info how to deflate it: https://unix.stackexchange.com/questions/22834/how-to-uncompress-zlib-data-in-unix

Something like the following will print the raw content, including the "$type $length\0" header:

perl -MCompress::Zlib -e 'undef $/; print uncompress(<>)' \
     < .git/objects/27/de0a1dd5a89a94990618632967a1c86a82d577

what happened is is the > part executes first and makes a new empty file to collect the output, but this process destroys the intended input data.

To fix: explicitly use a different file-name for the output.

for z in *.zlib; do; zlib-flate -uncompress < "$z" > "${z%.zlib}" ; done

here "${z%.zlib}" trims the .zlib off the end of the filenames.

use git cat-file -p SHA1 to view the file content of the blob.

The content of the file is this: blob XXX NULL Content

ex:

a.txt contain Hello World

The Content of the SHA-1 is:

blob 11\000Hello World and this strign is then GZipped

If you are on unix u can use this to deflate the content:
perl -MCompress::Zlib -e 'undef $/; print uncompress(<>)'