is it possible to add a comment to a diff file (unified)?

Question

I wonder if it's possible to add a certain amount of unparsed content to a diff file (unified) that is ignored as a comment.

One good use of this would be having git diffs augmented with important information such as from which branch is that diff from (especially when using the --full-index option, which merely displays the blob references).

Answer 1

The unified diff starts with two line header:

 --- from-file from-file-modification-time
 +++ to-file to-file-modification-time

Anything before this header is ignored, so you can add any comment here, for example:

 This may be some useful description of this patch that
 will be ignored by the diff/patch utility.
 --- a/foo  2002-02-21 23:30:39.942229878 -0800
 +++ b/foo  2002-02-21 23:30:50.442260588 -0800
 @@ -1,7 +1,6 @@
 -The Way that can be told of is not the eternal Way;
 -The name that can be named is not the eternal name.
  The Nameless is the origin of Heaven and Earth;
 -The Named is the mother of all things.
 +The named is the mother of all things.
 +
  Therefore let there always be non-being,
    so we may see their subtlety,
  And let there always be being,

Git itself uses this space before header for some metadata, for example:

diff --git a/foo b/foo
index 59a4d1f..e48dfe7 100644
--- a/foo
+++ b/foo

Answer 2

You can use # instead of - or + in the diff file for inline comments (recognizable by GNU patch), or add a comment at the begining mentioned by @JakubJirutka

 This may be some useful description of this patch that
 will be ignored by the diff/patch utility.
 --- a/foo  2002-02-21 23:30:39.942229878 -0800
 +++ b/foo  2002-02-21 23:30:50.442260588 -0800
 @@ -1,7 +1,6 @@
 # comments on the deletion of the two lines
 -The Way that can be told of is not the eternal Way;
 -The name that can be named is not the eternal name.
  The Nameless is the origin of Heaven and Earth;
 -The Named is the mother of all things.
 +The named is the mother of all things.
 +
  Therefore let there always be non-being,
    so we may see their subtlety,
  And let there always be being,

Answer 3

Anything after @@ on the same line is also ignored.

 --- a/foo  2002-02-21 23:30:39.942229878 -0800
 +++ b/foo  2002-02-21 23:30:50.442260588 -0800
 @@ -1,7 +1,6 @@ THIS IS A COMMENT THAT WILL BE IGNORED
 -The Way that can be told of is not the eternal Way;
 -The name that can be named is not the eternal name.
  The Nameless is the origin of Heaven and Earth;
 -The Named is the mother of all things.
 +The named is the mother of all things.
 +
  Therefore let there always be non-being,
    so we may see their subtlety,
  And let there always be being,