Do not allow linking with entry itself

Without this check, linking an entry with itself yields the following
error:

    ERROR[   1]: Entry is already borrowed: StoreId { base: Some("/home/m/.imag/store"), id: "notes/test" }
    ERROR[   2]: Error when calling retrieve() -- caused:
    ERROR[   3]: Error when calling get()

Which is semantically correct, but the user may get confused by that.

Instead, we print a nice error message that the entry cannot be linked
to itself.

This is not fixed in libimagentrylink itself, because libimagentrylink
cannot be called for the same entry.
If this would be possible, we would pass two `Entry` objects
mutably to the link functionality routines. This is not possible with
Rusts borrow semantics and therefor yields above error.

We compare strings to check whether the user accidentially linked an
entry with itself because we cannot get StoreIds from Entries because we
cannot get the Entry two times from the store in the first place. So
this is the best we have.

bin/core/imag-link/src/main.rs

@@ -61,6 +61,7 @@ use libimagrt::runtime::Runtime;
 use libimagrt::setup::generate_runtime_setup;
 use libimagstore::error::StoreError;
 use libimagstore::store::FileLockEntry;
+use libimagstore::storeid::StoreId;
 use libimagutil::warn_exit::warn_exit;
 use libimagutil::warn_result::*;
 
@@ -134,7 +135,16 @@ fn link_from_to<'a, I>(rt: &'a Runtime, from: &'a str, to: I)
                 .map_err_trace_exit_unwrap(1);
         } else {
             debug!("Linking internally: {:?} -> {:?}", from, entry);
-            let mut to_entry = match get_entry_by_name(rt, entry) {
+
+            let from_id = StoreId::new_baseless(PathBuf::from(from)).map_err_trace_exit_unwrap(1);
+            let entr_id = StoreId::new_baseless(PathBuf::from(entry)).map_err_trace_exit_unwrap(1);
+
+            if from_id == entr_id {
+                error!("Cannot link entry with itself. Exiting");
+                ::std::process::exit(1)
+            }
+
+            let mut to_entry = match rt.store().get(entr_id) {
                 Ok(Some(e)) => e,
                 Ok(None)    => {
                     warn!("No 'to' entry: {}", entry);