feat(changelog): support prepending entries to an existing changelog

git-cliff/src/args.rs

@@ -35,6 +35,9 @@ pub struct Opt {
 	/// Sets the tag for the latest version.
 	#[structopt(short, long, env, value_name = "TAG", allow_hyphen_values = true)]
 	pub tag:        Option<String>,
+	/// Prepend entries to the given changelog from stdin.
+	#[structopt(short, long)]
+	pub prepend:    bool,
 	/// Processes the commits starting from the latest tag.
 	#[structopt(short, long)]
 	pub latest:     bool,

git-cliff/src/changelog.rs

@@ -91,4 +91,18 @@ impl<'a> Changelog<'a> {
 		}
 		Ok(())
 	}
+
+	/// Generate changelog and prepend it to the given changelog.
+	pub fn prepend<W: Write>(
+		&self,
+		mut changelog: String,
+		out: &mut W,
+	) -> Result<()> {
+		if let Some(header) = &self.config.changelog.header {
+			changelog = changelog.replacen(header, "", 1);
+		}
+		self.generate(out)?;
+		write!(out, "{}", changelog)?;
+		Ok(())
+	}
 }

git-cliff/src/main.rs

@@ -9,7 +9,10 @@ use git_cliff_core::error::Result;
 use git_cliff_core::release::Release;
 use git_cliff_core::repo::Repository;
 use std::env;
-use std::io;
+use std::io::{
+	self,
+	Read,
+};
 use structopt::StructOpt;
 #[macro_use]
 extern crate log;
@@ -27,6 +30,8 @@ fn main() -> Result<()> {
 	if args.strip {
 		config.changelog.header = None;
 		config.changelog.footer = None;
+	} else if args.prepend {
+		config.changelog.footer = None;
 	}
 
 	let repository =
@@ -77,5 +82,12 @@ fn main() -> Result<()> {
 		}
 	}
 
-	Changelog::new(releases, &config)?.generate(&mut io::stdout())
+	let changelog = Changelog::new(releases, &config)?;
+	if args.prepend {
+		let mut buffer = String::new();
+		io::stdin().read_to_string(&mut buffer)?;
+		changelog.prepend(buffer, &mut io::stdout())
+	} else {
+		changelog.generate(&mut io::stdout())
+	}
 }