Add command line parameter to specify log file

I had the logs of my debug helix mixed in with the logs from the
production helix.

Add a `--log` command line argument to redirect any logs to other
files, making my debugging easier :-)

helix-term/src/args.rs

@@ -14,6 +14,7 @@ pub struct Args {
     pub build_grammars: bool,
     pub split: Option<Layout>,
     pub verbosity: u64,
+    pub log_file: Option<PathBuf>,
     pub config_file: Option<PathBuf>,
     pub files: Vec<(PathBuf, Position)>,
 }
@@ -48,6 +49,10 @@ impl Args {
                     Some(path) => args.config_file = Some(path.into()),
                     None => anyhow::bail!("--config must specify a path to read"),
                 },
+                "--log" => match argv.next().as_deref() {
+                    Some(path) => args.log_file = Some(path.into()),
+                    None => anyhow::bail!("--log must specify a path to write"),
+                },
                 arg if arg.starts_with("--") => {
                     anyhow::bail!("unexpected double dash argument: {}", arg)
                 }

helix-term/src/main.rs

@@ -67,6 +67,7 @@ FLAGS:
     -g, --grammar {{fetch|build}}    Fetches or builds tree-sitter grammars listed in languages.toml
     -c, --config <file>            Specifies a file to use for configuration
     -v                             Increases logging verbosity each use for up to 3 times
+    --log                          Specifies a file to use for logging
                                    (default file: {})
     -V, --version                  Prints version information
     --vsplit                       Splits all given files vertically into different windows
@@ -114,6 +115,7 @@ FLAGS:
         return Ok(0);
     }
 
+    let logpath = args.log_file.as_ref().cloned().unwrap_or(logpath);
     setup_logging(logpath, args.verbosity).context("failed to initialize logging")?;
 
     let config_dir = helix_loader::config_dir();