blame: add the ability to ignore commits and their changes

Commits that make formatting changes or function renames are often not
interesting when blaming a file.  A user may deem such a commit as 'not
interesting' and want to ignore and its changes it when assigning blame.

For example, say a file has the following git history / rev-list:

---O---A---X---B---C---D---Y---E---F

Commits X and Y both touch a particular line, and the other commits do
not:

X: "Take a third parameter"
-MyFunc(1, 2);
+MyFunc(1, 2, 3);

Y: "Remove camelcase"
-MyFunc(1, 2, 3);
+my_func(1, 2, 3);

git-blame will blame Y for the change.  I'd like to be able to ignore Y:
both the existence of the commit as well as any changes it made.  This
differs from -S rev-list, which specifies the list of commits to
process for the blame.  We would still process Y, but just don't let the
blame 'stick.'

This patch adds the ability for users to ignore a revision with
--ignore-rev=rev, which may be repeated.  They can specify a set of
files of full object names of revs, e.g. SHA-1 hashes, one per line.  A
single file may be specified with the blame.ignoreRevFile config option
or with --ignore-rev-file=file.  Both the config option and the command
line option may be repeated multiple times.  An empty file name "" will
clear the list of revs from previously processed files.  Config options
are processed before command line options.

For a typical use case, projects will maintain the file containing
revisions for commits that perform mass reformatting, and their users
have the option to ignore all of the commits in that file.

Additionally, a user can use the --ignore-rev option for one-off
investigation.  To go back to the example above, X was a substantive
change to the function, but not the change the user is interested in.
The user inspected X, but wanted to find the previous change to that
line - perhaps a commit that introduced that function call.

To make this work, we can't simply remove all ignored commits from the
rev-list.  We need to diff the changes introduced by Y so that we can
ignore them.  We let the blames get passed to Y, just like when
processing normally.  When Y is the target, we make sure that Y does not
*keep* any blames.  Any changes that Y is responsible for get passed to
its parent.  Note we make one pass through all of the scapegoats
(parents) to attempt to pass blame normally; we don't know if we *need*
to ignore the commit until we've checked all of the parents.

The blame_entry will get passed up the tree until we find a commit that
has a diff chunk that affects those lines.

One issue is that the ignored commit *did* make some change, and there is
no general solution to finding the line in the parent commit that
corresponds to a given line in the ignored commit.  That makes it hard
to attribute a particular line within an ignored commit's diff
correctly.

For example, the parent of an ignored commit has this, say at line 11:

commit-a 11) #include "a.h"
commit-b 12) #include "b.h"

Commit X, which we will ignore, swaps these lines:

commit-X 11) #include "b.h"
commit-X 12) #include "a.h"

We can pass that blame entry to the parent, but line 11 will be
attributed to commit A, even though "include b.h" came from commit B.
The blame mechanism will be looking at the parent's view of the file at
line number 11.

ignore_blame_entry() is set up to allow alternative algorithms for
guessing per-line blames.  Any line that is not attributed to the parent
will continue to be blamed on the ignored commit as if that commit was
not ignored.  Upcoming patches have the ability to detect these lines
and mark them in the blame output.

The existing algorithm is simple: blame each line on the corresponding
line in the parent's diff chunk.  Any lines beyond that stay with the
target.

For example, the parent of an ignored commit has this, say at line 11:

commit-a 11) void new_func_1(void *x, void *y);
commit-b 12) void new_func_2(void *x, void *y);
commit-c 13) some_line_c
commit-d 14) some_line_d

After a commit 'X', we have:

commit-X 11) void new_func_1(void *x,
commit-X 12)                 void *y);
commit-X 13) void new_func_2(void *x,
commit-X 14)                 void *y);
commit-c 15) some_line_c
commit-d 16) some_line_d

Commit X nets two additionally lines: 13 and 14.  The current
guess_line_blames() algorithm will not attribute these to the parent,
whose diff chunk is only two lines - not four.

When we ignore with the current algorithm, we get:

commit-a 11) void new_func_1(void *x,
commit-b 12)                 void *y);
commit-X 13) void new_func_2(void *x,
commit-X 14)                 void *y);
commit-c 15) some_line_c
commit-d 16) some_line_d

Note that line 12 was blamed on B, though B was the commit for
new_func_2(), not new_func_1().  Even when guess_line_blames() finds a
line in the parent, it may still be incorrect.

Signed-off-by: Barret Rhoden <brho@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/Documentation/config/blame.txt b/Documentation/config/blame.txt
index 67b5c1d1e0..4da2788f30 100644
--- a/Documentation/config/blame.txt
+++ b/Documentation/config/blame.txt
@@ -19,3 +19,10 @@ blame.showEmail::
 blame.showRoot::
 	Do not treat root commits as boundaries in linkgit:git-blame[1].
 	This option defaults to false.
+
+blame.ignoreRevsFile::
+	Ignore revisions listed in the file, one unabbreviated object name per
+	line, in linkgit:git-blame[1].  Whitespace and comments beginning with
+	`#` are ignored.  This option may be repeated multiple times.  Empty
+	file names will reset the list of ignored revisions.  This option will
+	be handled before the command line option `--ignore-revs-file`.