rc tools git: command for easy recursive blaming

Our ":git blame" annotates each line with the most recent commit.
However often a line has been modified by several commits.

Introduce ":git blame-jump" which shows the commit that added the
line at cursor. Crucially, it works also in Git diff buffers, so it
can be used recursively to find the full history of a line.

To do the recursive blame from a diff, I need to navigate to the
old (deleted) version of a line. Since old and new line are usually
neighbors. Speed up the common scenario of finding the old version
by making ":git blame-jump" jump to the new version. This means the
initial diff view might not include the commit message etc. Compensate
this by showing the commit's date+author+subject in the status line.

Here are some test cases.
- run blame-jump after "git blame"
- create an uncommitted or unsaved line, run "git blame" and
  "blame-jump" on the uncommitted line
- run blame-jump without running "git blame"
- run blame-jump in "git show"
- run blame-jump in "git diff HEAD"
- run blame-jump in "git diff --cached"
- run blame-jump in "git diff" (YMMV if there are cached changes,
  could fix that)

Naming: there are some similar commands in the wild [1];
they are usually called "show-blamed" or similar, but they
don't jump to the corresponding line.  Also our list of git
commands is getting a bit messy (especially the undocumented
show-diff/hide-diff/next-hunk/prev-hunk; subject first naming seems
better).

[1]: https://github.com/robertmeta/kak/blob/f6e78ec4c0eeccd091e6275828234d98e6aa3a7f/kakrc#L423

Future work: to go back to the previously-blamed commit we need to
have had the foresight to use "rename-buffer". Perhaps we want to
add some kind of buffer stack (like Magit does for example).

1 files changed, 16 insertions, 0 deletions

diff --git a/rc/filetype/diff-parse.pl b/rc/filetype/diff-parse.pl
index f7858eb7..203e9b19 100755
--- a/rc/filetype/diff-parse.pl
+++ b/rc/filetype/diff-parse.pl
@@ -39,11 +39,17 @@ while (defined $ARGV[0]) {
 # Inputs
 our $directory = $ENV{PWD};
 our $strip;
+our $in_file;
+our $in_file_line;
 our $version = "+";
 
 eval $begin if defined $begin;
 
+$in_file = "$directory/$in_file" if defined $in_file;
+
 # Outputs
+our $diff_line = 0;
+our $commit;
 our $file;
 our $file_line;
 our $diff_line_text;
@@ -83,8 +89,13 @@ sub strip {
 }
 
 while (<STDIN>) {
+    $diff_line++;
     s/^(> )*//g;
     $diff_line_text = $_;
+    if (m{^commit (\w+)}) {
+        $commit = $1;
+        next;
+    }
     if (m{^diff\b}) {
         $state = "header";
         $is_recursive_diff = 1;
@@ -116,6 +127,11 @@ while (<STDIN>) {
            $other_file_line++ if defined $other_file_line;
         }
     }
+    if (defined $in_file and defined $file and $file eq $in_file) {
+        if (defined $in_file_line and defined $file_line and $file_line >= $in_file_line) {
+            last;
+        }
+    }
 }
 if (not defined $file) {
     $file = ($fallback_file or $other_file);