Load a commit.template message if one is configured (gitui-org#546)

If a commit message template is set with...

    git config commit.template /path/to/some/file.txt

...this fills the commit message box (and therefore, later, the external
editor) with the contents of that file, as long as you don't have something
more interesting typed there already.

Like the `git commit` command, this doesn't let you create a commit that has
just the template as its message, unchanged.  Unlike the `git commit` command,
not being able to read the template file is not fatal: specifically, if the
commit.template setting is pointed at a file that does not exist, this does
nothing.

Resolves gitui-org#546.

src/components/commit.rs

@@ -13,12 +13,12 @@ use crate::{
 use anyhow::Result;
 use asyncgit::{
     cached,
-    sync::{self, CommitId, HookResult},
+    sync::{self, utils::get_config_string, CommitId, HookResult},
     CWD,
 };
 use crossterm::event::Event;
 use std::{
-    fs::File,
+    fs::{read_to_string, File},
     io::{Read, Write},
     path::PathBuf,
 };
@@ -35,6 +35,7 @@ pub struct CommitComponent {
     queue: Queue,
     key_config: SharedKeyConfig,
     git_branch_name: cached::BranchName,
+    commit_template: Option<String>,
 }
 
 impl DrawableComponent for CommitComponent {
@@ -129,6 +130,13 @@ impl Component for CommitComponent {
 
         self.input
             .set_title(strings::commit_title(&self.key_config));
+
+        if self.is_empty() {
+            if let Some(s) = &self.commit_template {
+                self.input.set_text(s.clone());
+            }
+        }
+
         self.input.show()?;
 
         Ok(())
@@ -154,12 +162,22 @@ impl CommitComponent {
             ),
             key_config,
             git_branch_name: cached::BranchName::new(CWD),
+            commit_template: None,
         }
     }
 
     ///
     pub fn update(&mut self) -> Result<()> {
         self.git_branch_name.lookup().map(Some).unwrap_or(None);
+
+        self.commit_template.get_or_insert_with(|| {
+            get_config_string(CWD, "commit.template")
+                .ok()
+                .unwrap_or(None)
+                .and_then(|path| read_to_string(path).ok())
+                .unwrap_or_else(String::new)
+        });
+
         Ok(())
     }
 
@@ -291,13 +309,22 @@ impl CommitComponent {
     }
 
     fn can_commit(&self) -> bool {
-        !self.input.get_text().is_empty()
+        !self.is_empty() && self.is_changed()
     }
 
     fn can_amend(&self) -> bool {
         self.amend.is_none()
             && sync::get_head(CWD).is_ok()
-            && self.input.get_text().is_empty()
+            && (self.is_empty() || !self.is_changed())
+    }
+
+    fn is_empty(&self) -> bool {
+        self.input.get_text().is_empty()
+    }
+
+    fn is_changed(&self) -> bool {
+        Some(self.input.get_text().trim())
+            != self.commit_template.as_ref().map(|s| s.trim())
     }
 
     fn amend(&mut self) -> Result<()> {