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Git Community Book
The data part is just zlib stream for non-delta object types; for the two delta object representations, the data portion
contains something that identifies which base object this delta representation depends on, and the delta to apply on the
base object to resurrect this object. ref-delta uses 20-byte hash of the base object at the beginning of data, while ofsdelta stores an offset within the same packfile to identify the base object. In either case, two important constraints a
reimplementor must adhere to are:
• delta representation must be based on some other object within the same packfile;
• the base object must be of the same underlying type (blob, tree, commit or tag);
RAW GIT
Here we will take a look at how to manipulate git at a more raw level, in case you would like to write a tool that generates
new blobs, trees or commits in a more artificial way. If you want to write a script that uses more low-level git plumbing to
do something new, here are some of the tools you'll need.
Creating Blobs
Creating a blob in your Git repository and getting a SHA back is pretty easy. The git hash-object command is all you'll
need. To create a blob object from an existing file, just run it with the '-w' option (which tells it to write the blob, not just
compute the SHA).
$ git hash-object -w myfile.txt
6ff87c4664981e4397625791c8ea3bbb5f2279a3
$ git hash-object -w myfile2.txt
3bb0e8592a41ae3185ee32266c860714980dbed7
128