warn: warn about RUSTIC_APP.config() vs self usage

In commands that have configurations from
the ENV and the TOML (on top of CLI),
a confusion between RUSTIC_APP.config()
and &self (see `inner_run` impls) could
lead to parts of the user configured options
to be ignored.

For now we warn about this in the code.
But some design change should be decided
in order to avoid these risks in the future.

src/commands/forget.rs

@@ -98,6 +98,9 @@ impl Runnable for ForgetCmd {
 }
 
 impl ForgetCmd {
+    /// be careful about self VS RUSTIC_APP.config() usage
+    /// only the RUSTIC_APP.config() involves the TOML and ENV merged configurations
+    /// see https://github.com/rustic-rs/rustic/issues/1242
     fn inner_run(&self) -> Result<()> {
         let config = RUSTIC_APP.config();
         let repo = open_repository(&config.repository)?;

src/commands/webdav.rs

@@ -66,6 +66,9 @@ impl Runnable for WebDavCmd {
 }
 
 impl WebDavCmd {
+    /// be careful about self VS RUSTIC_APP.config() usage
+    /// only the RUSTIC_APP.config() involves the TOML and ENV merged configurations
+    /// see https://github.com/rustic-rs/rustic/issues/1242
     fn inner_run(&self) -> Result<()> {
         let config = RUSTIC_APP.config();
         let repo = open_repository_indexed(&config.repository)?;