refactor: improve help text for revision-range, default to HEAD

Just took the text help from `man git-shortlog` on `<revision-range>`,
since that is exactly where it is being called.

README.md

@@ -27,15 +27,17 @@ cargo install git-stats --locked
 ## Usage
 
 ```
-Usage: git-stats [OPTIONS] <REV_RANGE>
+$ git stats -h
+A script for grabbing more thorough shortlog stats
+
+Usage: git-stats [OPTIONS] [revision-range]
 
 Arguments:
-  <REV_RANGE>  The revision range to consider
+  [revision-range]  Show only commits in the specified revision range [default: HEAD]
 
 Options:
   -v, --verbose...  More output per occurrence
   -q, --quiet...    Less output per occurrence
-  -h, --help        Print help information
+  -h, --help        Print help information (use `--help` for more detail)
   -V, --version     Print version information
-
 ```

src/main.rs

@@ -11,7 +11,13 @@ use xshell::{cmd, Shell};
 #[derive(Parser)]
 #[command(author, version, about, long_about = None)]
 struct Cli {
-    /// The revision range to consider
+    #[arg(name = "revision-range", default_value = "HEAD")]
+    /// Show only commits in the specified revision range.
+    ///
+    /// When no <revision-range> is specified, it defaults to HEAD (i.e. the whole history leading
+    /// to the current commit). origin..HEAD specifies all the commits reachable from the current
+    /// commit (i.e. HEAD), but not from origin. For a complete list of ways to spell
+    /// [revision-range], see the "Specifying Ranges" section of gitrevisions(7).
     rev_range: String,
     #[command(flatten)]
     verbose: Verbosity<WarnLevel>,