diffcore: add a pickaxe option to find a specific blob

Sometimes users are given a hash of an object and they want to
identify it further (ex.: Use verify-pack to find the largest blobs,
but what are these? or [1])

One might be tempted to extend git-describe to also work with blobs,
such that `git describe <blob-id>` gives a description as
'<commit-ish>:<path>'.  This was implemented at [2]; as seen by the sheer
number of responses (>110), it turns out this is tricky to get right.
The hard part to get right is picking the correct 'commit-ish' as that
could be the commit that (re-)introduced the blob or the blob that
removed the blob; the blob could exist in different branches.

Junio hinted at a different approach of solving this problem, which this
patch implements. Teach the diff machinery another flag for restricting
the information to what is shown. For example:

    $ ./git log --oneline --find-object=v2.0.0:Makefile
    b2feb64309 Revert the whole "ask curl-config" topic for now
    47fbfded53 i18n: only extract comments marked with "TRANSLATORS:"

we observe that the Makefile as shipped with 2.0 was appeared in
v1.9.2-471-g47fbfded53 and in v2.0.0-rc1-5-gb2feb6430b.  The
reason why these commits both occur prior to v2.0.0 are evil
merges that are not found using this new mechanism.

[1] https://stackoverflow.com/questions/223678/which-commit-has-this-blob
[2] https://public-inbox.org/git/20171028004419.10139-1-sbeller@google.com/

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index dd0dba5b1d..f9cf85db05 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -492,6 +492,15 @@ occurrences of that string did not change).
 See the 'pickaxe' entry in linkgit:gitdiffcore[7] for more
 information.
 
+--find-object=<object-id>::
+	Look for differences that change the number of occurrences of
+	the specified object. Similar to `-S`, just the argument is different
+	in that it doesn't search for a specific string but for a specific
+	object id.
++
+The object can be a blob or a submodule commit. It implies the `-t` option in
+`git-log` to also find trees.
+
 --pickaxe-all::
 	When `-S` or `-G` finds a change, show all the changes in that
 	changeset, not just the files that contain the change
@@ -500,6 +509,7 @@ information.
 --pickaxe-regex::
 	Treat the <string> given to `-S` as an extended POSIX regular
 	expression to match.
+
 endif::git-format-patch[]
 
 -O<orderfile>::