fish: edit: write to the real file name

When saving, resolve the path of the file being edited and use that as
real target to write to. Otherwise, if the file name is a symlink then
it will be replaced by a regular file with the new content, leaving the
old file untouched.

Extend test-edit.sh to check for this situation.

fish/test-edit.sh

@@ -43,6 +43,12 @@ cat /file.txt
 stat /file.txt | grep mode:
 stat /file.txt | grep uid:
 stat /file.txt | grep gid:
+echo ==========
+write /file-2.txt "symlink test\n"
+ln-s /file-2.txt /symlink-2.txt
+edit /symlink-2.txt
+is-symlink /symlink-2.txt
+cat /symlink-2.txt
 EOF
 )
 
@@ -51,7 +57,11 @@ second line of text
 
 mode: 33152
 uid: 10
-gid: 11" ]; then
+gid: 11
+==========
+true
+symlink test
+second line of text" ]; then
     echo "$0: error: output of guestfish after edit command did not match expected output"
     echo "$output"
     exit 1