rc git: teach "git apply" to work on content, not just diffs

Staging/unstaging/reverting (parts of) the current buffer's file can
be a common use case.

Today "git apply" can do that based on a selection within a diff.
When the selection is on uncommitted content, we can probably assume
that the intent is to use the part of the selection that overlaps
with the +-side of "git diff" (or "git diff --cached" for
"git apply --cached").

Make "git apply" treat selections as content if the buffile is
tracked by Git. This differentiator is not perfect but I don't know
why anyone would want to use the existing "git apply" semantics on
a tracked file. Maybe we should pick a different name.

This feature couples well with "git show-diff", which shows all
lines with unstaged changes (in future it should probably show staged
changes as well).

Whereas on diffs, "git apply" stages the entire hunk if the selection
contains no newline, this does not happen when operating on content.
I didn't yet try implementing that.  I guess the hunks are not as
explicit here.

Closes #5225

1 files changed, 15 insertions, 5 deletions

diff --git a/rc/tools/patch-range.pl b/rc/tools/patch-range.pl
index 2d54ab26..01611abe 100755
--- a/rc/tools/patch-range.pl
+++ b/rc/tools/patch-range.pl
@@ -9,6 +9,12 @@ if ($ARGV[0] eq "-print-remaining-diff") {
     shift @ARGV;
 }
 
+my $line_number_kind = "diff";
+if ($ARGV[0] eq "-line-numbers-from-new-file") {
+    $line_number_kind = "new-file";
+    shift @ARGV;
+}
+
 my $min_line = $ARGV[0];
 shift @ARGV;
 my $max_line = $ARGV[0];
@@ -22,7 +28,7 @@ if (defined $ARGV[0] and $ARGV[0] =~ m{^[^-]}) {
 }
 my $reverse = grep /^(--reverse|-R)$/, @ARGV;
 
-my $lineno = 0;
+my $lineno = $line_number_kind eq "diff" ? 0 : undef;
 my $original = "";
 my $diff_header = "";
 my $wheat = "";
@@ -63,9 +69,9 @@ sub finish_hunk {
 }
 
 while (<STDIN>) {
-    ++$lineno;
+    ++$lineno if $line_number_kind eq "diff";
     $original .= $_;
-    if (m{^diff}) {
+    if (m{^diff} || (not defined $state and m{^---})) {
         finish_hunk();
         $state = "diff header";
         $diff_header = "";
@@ -74,7 +80,8 @@ while (<STDIN>) {
         $signature .= $_ if $print_remaining;
         next;
     }
-    if (m{^@@ -\d+(?:,(\d)+)? \+\d+(?:,\d+)? @@}) {
+    if (m{^@@ -\d+(?:,(\d)+)? \+(\d+)(?:,\d+)? @@}) {
+        $lineno = $2 - 1 if $line_number_kind eq "new-file";
         $hunk_remaining_lines = $1 or 1;
         finish_hunk();
         $state = "diff hunk";
@@ -96,8 +103,11 @@ while (<STDIN>) {
         $signature .= $_ if $print_remaining;
         next;
     }
+    ++$lineno if $line_number_kind eq "new-file" && m{^[ +]};
     --$hunk_remaining_lines if m{^[ -]};
-    my $include = m{^ } || ($lineno >= $min_line && $lineno <= $max_line);
+    my $include = m{^ } ||
+        ($lineno >= $min_line && $lineno <= $max_line) ||
+        ($line_number_kind eq "new-file" && m{^-} && $lineno == $min_line - 1);
     if ($include) {
         $hunk_wheat .= $_;
         if ($print_remaining) {