templates: create new git_format_patch_email_headers template

With this template, a 'git format-patch' compatible
email message can be generated using something like:

jj show --git --template git_format_patch_email_headers <rev>

cli/src/config/templates.toml

@@ -402,3 +402,21 @@ coalesce(
   "o",
 )
 '''
+
+# The first line is a fixed magic date string used by
+# programs like file(1) to identify that this is output from
+# 'git format-patch'.
+# See https://git-scm.com/docs/git-format-patch#_description
+git_format_patch_email_headers = '''
+  concat(
+    "From " ++ commit_id ++ " Mon Sep 17 00:00:00 2001\n",
+    "From: " ++ author ++ "\n",
+    "Date: " ++ author.timestamp().format("%a, %-e %b %Y %T %z") ++ "\n",
+    "Subject: [PATCH] " ++ description.first_line() ++ "\n",
+    "\n",
+    description.remove_prefix(description.first_line()).trim_start(),
+    "---\n",
+    indent(" ", diff.stat()),
+    "\n"
+  )
+'''

cli/tests/test_commit_template.rs

@@ -1609,3 +1609,49 @@ fn test_signature_templates() {
     [EOF]
     ");
 }
+
+#[test]
+fn test_log_git_format_patch_template() {
+    let test_env = TestEnvironment::default();
+    test_env.run_jj_in(".", ["git", "init", "repo"]).success();
+    let work_dir = test_env.work_dir("repo");
+
+    work_dir.write_file("file1", "foo\n");
+    work_dir.write_file("file2", "bar\n");
+    work_dir
+        .run_jj([
+            "new",
+            "-m",
+            "some change\n\nmultiline desc\nsecond line\n\nwith blanks\n",
+        ])
+        .success();
+    work_dir.remove_file("file1");
+    work_dir.write_file("file2", "modified\n");
+    work_dir.write_file("file3", "new\n");
+
+    let output = work_dir.run_jj([
+        "log",
+        "--no-graph",
+        "-T",
+        "git_format_patch_email_headers",
+        "-r@",
+    ]);
+    insta::assert_snapshot!(output, @r"
+    From 993219c0b219b5eeac6303b5cb2bf943ea719672 Mon Sep 17 00:00:00 2001
+    From: Test User <test.user@example.com>
+    Date: Sat, 3 Feb 2001 04:05:08 +0700
+    Subject: [PATCH] some change
+
+    multiline desc
+    second line
+
+    with blanks
+    ---
+     file1 | 1 -
+     file2 | 2 +-
+     file3 | 1 +
+     3 files changed, 2 insertions(+), 2 deletions(-)
+
+    [EOF]
+    ");
+}

cli/tests/test_completion.rs

@@ -766,6 +766,7 @@ fn test_template_alias() {
     commit_summary_separator
     description_placeholder
     email_placeholder
+    git_format_patch_email_headers
     name_placeholder
     [EOF]
     ");

cli/tests/test_evolog_command.rs

@@ -361,6 +361,7 @@ fn test_evolog_with_no_template() {
     - commit_summary_separator
     - description_placeholder
     - email_placeholder
+    - git_format_patch_email_headers
     - name_placeholder
     [EOF]
     [exit status: 2]

cli/tests/test_log_command.rs

@@ -63,6 +63,7 @@ fn test_log_with_no_template() {
     - commit_summary_separator
     - description_placeholder
     - email_placeholder
+    - git_format_patch_email_headers
     - name_placeholder
     [EOF]
     [exit status: 2]

cli/tests/test_operations.rs

@@ -182,6 +182,7 @@ fn test_op_log_with_no_template() {
     - commit_summary_separator
     - description_placeholder
     - email_placeholder
+    - git_format_patch_email_headers
     - name_placeholder
     [EOF]
     [exit status: 2]

cli/tests/test_show_command.rs

@@ -288,6 +288,7 @@ fn test_show_with_no_template() {
     - commit_summary_separator
     - description_placeholder
     - email_placeholder
+    - git_format_patch_email_headers
     - name_placeholder
     [EOF]
     [exit status: 2]

demos/demo_git_compat.sh

@@ -29,6 +29,9 @@ run_command "jj diff -r b1"
 blank
 run_command "jj diff -r b3"
 
+comment "We can generate a 'git format-patch' compatible diff"
+run_command "jj show --git --template git_format_patch_email_headers"
+
 comment "The repo is backed by the actual Git repo:"
 run_command "git --git-dir=.jj/repo/store/git log --graph --all --decorate --oneline"