blame: respect .git-blame-ignore-revs automatically

Modify `git blame` to automatically respect a `.git-blame-ignore-revs`
file if it exists in the repository. This file is used by many projects
to ignore non-functional changes, such as reformatting or large-scale
refactoring, when generating blame information.

Before this change, users had to manually specify the file with the
`--ignore-revs-file` option. This update streamlines the process by
automatically detecting the `.git-blame-ignore-revs` file, reducing
manual effort.

This change aligns with the standardized practice in many repositories
and simplifies the workflow for users.

Signed-off-by: Abhijeetsingh Meena <abhijeet040403@gmail.com>

builtin/blame.c

@@ -1105,6 +1105,14 @@ int cmd_blame(int argc,
 		add_pending_object(&revs, &head_commit->object, "HEAD");
 	}
 
+	/*
+	* By default, add .git-blame-ignore-revs to the list of files
+	* containing revisions to ignore if it exists.
+	*/
+	if (access(".git-blame-ignore-revs", F_OK) == 0) {
+		string_list_append(&ignore_revs_file_list, ".git-blame-ignore-revs");
+	}
+
 	init_scoreboard(&sb);
 	sb.revs = &revs;
 	sb.contents_from = contents_from;

t/t8015-blame-default-ignore-revs.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+
+test_description='default revisions to ignore when blaming'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+test_expect_success 'blame: default-ignore-revs-file' '
+    test_commit first-commit hello.txt hello &&
+
+    echo world >>hello.txt &&
+    test_commit second-commit hello.txt &&
+
+    sed "1s/hello/hi/" <hello.txt > hello.txt.tmp &&
+    mv hello.txt.tmp hello.txt &&
+    test_commit third-commit hello.txt &&
+
+    git rev-parse HEAD >ignored-file &&
+    git blame --ignore-revs-file=ignored-file hello.txt >expect &&
+    git rev-parse HEAD >.git-blame-ignore-revs &&
+    git blame hello.txt >actual &&
+
+    test_cmp expect actual
+'
+
+test_done