log: add --count flag

Inspired by @indirect's talk at JJ Con (https://www.youtube.com/watch?v=ZnTNFIMjDwg)

Author: gechr

CHANGELOG.md

@@ -101,6 +101,9 @@ should not be broken.
 * Added `join()` template function. This is different from `separate()` in that
   it adds a separator between all arguments, even if empty.
 
+* `jj log` now supports a `--count` flag to print the number of commits instead
+  of displaying them.
+
 ### Fixed bugs
 
 * `jj fix` now prints a warning if a tool failed to run on a file.

cli/src/commands/log.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::cmp::min;
+
 use clap_complete::ArgValueCandidates;
 use clap_complete::ArgValueCompleter;
 use itertools::Itertools as _;
@@ -116,6 +118,9 @@ pub(crate) struct LogArgs {
     /// Show patch
     #[arg(long, short = 'p')]
     patch: bool,
+    /// Print the number of commits instead of showing them
+    #[arg(long, conflicts_with_all = ["DiffFormatArgs", "no_graph", "patch", "reversed", "template"])]
+    count: bool,
     #[command(flatten)]
     diff_format: DiffFormatArgs,
 }
@@ -150,12 +155,32 @@ pub(crate) fn cmd_log(
         }
         expression
     };
+
+    let revset = revset_expression.evaluate()?;
+
+    if args.count {
+        let (lower, upper) = revset.count_estimate()?;
+        let limit = args.limit.unwrap_or(usize::MAX);
+        let count = if limit <= lower {
+            limit
+        } else if upper == Some(lower) {
+            min(lower, limit)
+        } else {
+            revset
+                .iter()
+                .take(limit)
+                .process_results(|iter| iter.count())?
+        };
+        let mut formatter = ui.stdout_formatter();
+        writeln!(formatter, "{count}")?;
+        return Ok(());
+    }
+
     let prio_revset = settings.get_string("revsets.log-graph-prioritize")?;
     let prio_revset = workspace_command.parse_revset(ui, &RevisionArg::from(prio_revset))?;
 
     let repo = workspace_command.repo();
     let matcher = fileset_expression.to_matcher();
-    let revset = revset_expression.evaluate()?;
 
     let store = repo.store();
     let diff_renderer = workspace_command.diff_renderer_for_log(&args.diff_format, args.patch)?;

cli/tests/cli-reference@.md.snap

@@ -1763,6 +1763,7 @@ The working-copy commit is indicated by a `@` symbol in the graph. [Immutable re
 
    [`jj help -k templates`]: https://docs.jj-vcs.dev/latest/templates/
 * `-p`, `--patch` — Show patch
+* `--count` — Print the number of commits instead of showing them
 * `-s`, `--summary` — For each path, show only whether it was modified, added, or deleted
 * `--stat` — Show a histogram of the changes
 * `--types` — For each path, show only its type before and after

cli/tests/test_log_command.rs

@@ -1757,3 +1757,34 @@ fn test_log_anonymize() {
     [EOF]
     ");
 }
+
+#[test]
+fn test_log_count() {
+    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.run_jj(["describe", "-m", "first"]).success();
+    work_dir.run_jj(["new", "-m", "second"]).success();
+    work_dir.write_file("file2", "bar\n");
+    work_dir.run_jj(["new", "-m", "third"]).success();
+
+    let output = work_dir.run_jj(["log", "--count"]);
+    insta::assert_snapshot!(output, @r"
+    4
+    [EOF]
+    ");
+
+    let output = work_dir.run_jj(["log", "--count", "-r", "all() ~ root()"]);
+    insta::assert_snapshot!(output, @r"
+    3
+    [EOF]
+    ");
+
+    let output = work_dir.run_jj(["log", "--count", "--limit", "2"]);
+    insta::assert_snapshot!(output, @r"
+    2
+    [EOF]
+    ");
+}