Add since-last-tag command

src/main.rs

@@ -54,6 +54,18 @@ enum Subcommand {
         #[clap(flatten)]
         files: FileSelection,
     },
+    /// Select the (local) mainline branch and the most recent tag.
+    ///
+    /// Many Git projects use tags to manage project releases and/or milestones. This command is
+    /// useful for viewing the set of changes since the most recent tag. This can be helpful when
+    /// for example, analyzing changes for determining which SemVer version to use for a next
+    /// release, performing risk assessments, or writing release notes.
+    SinceLastTag {
+        #[clap(long, short)]
+        base: Option<String>,
+        #[clap(flatten)]
+        files: FileSelection,
+    },
     /// Select a custom set of commit-ish refs.
     Select {
         /// Additional branches to include.
@@ -150,35 +162,40 @@ fn main() {
 
             Ok(branches)
         };
+
+        let fallback_base = || -> git_glimpse::Result<_> {
+            let base = git_config("glimpse.base")?;
+            let base = base.unwrap_or_else(|| {
+                let default = "main";
+                log::debug!(
+                    "no base branch specified in command line or configuration, falling back to \
+                    {default:?}"
+                );
+                default.to_owned()
+            });
+            Ok(base)
+        };
+
         let (branches, files) = match subcommand {
             Subcommand::Stack {
                 base,
                 config,
                 files: FileSelection { files },
             } => {
-                let specified_base = base
-                    .map(Ok)
-                    .or_else(|| git_config("glimpse.base").transpose())
-                    .transpose()?;
-                let base = specified_base.as_deref().unwrap_or_else(|| {
-                    let default = "main";
-                    log::debug!("no base branch specified in command line or configuration, falling back to {default:?}");
-                    default
-                });
-
+                let base = base.map(Ok).unwrap_or_else(fallback_base)?;
                 let branches = if let Some(current_branch) = current_branch()? {
                     let mut config = config;
                     if current_branch == base {
                         config.select_upstreams = true;
                     }
                     branches(&config, &|cmd| {
                         if base != current_branch {
-                            cmd.arg(base);
+                            cmd.arg(&base);
                         }
                         cmd.arg(&current_branch).arg("--all")
                     })?
                 } else {
-                    let mut branches = branches(&config, &|cmd| cmd.arg(base).arg("--all"))?;
+                    let mut branches = branches(&config, &|cmd| cmd.arg(&base).arg("--all"))?;
                     branches.push("HEAD".to_owned());
                     branches
                 };
@@ -188,6 +205,21 @@ fn main() {
                 config,
                 files: FileSelection { files },
             } => (branches(&config, &|cmd| cmd)?, files),
+            Subcommand::SinceLastTag {
+                base,
+                files: FileSelection { files },
+            } => {
+                let base = base.map(Ok).unwrap_or_else(fallback_base)?;
+                let branches = branches(
+                    &PresetConfig {
+                        select_upstreams: false,
+                        select_pushes: false,
+                        select_last_tag: true,
+                    },
+                    &|cmd| cmd.arg(&base),
+                )?;
+                (branches, files)
+            }
             Subcommand::Select {
                 branches,
                 files: FileSelection { files },