gptel: Include full file path in context (karthink#863)

* gptel.el (gptel--insert-file-string): Include the full file path
in the context for included files.  The file name is passed
through `abbreviate-file-name'.

* NEWS (New features and UI changes): Mention change and reason.

Author: karthink

NEWS

@@ -13,6 +13,14 @@
 
 ** New features and UI changes
 
+- When including a file in the context, the abbreviated full path of
+  the file is included is now included instead of the basename.
+  Specifically, =/home/user/path/to/file= is included as
+  =~/path/to/file=.  This is to provide additional context for LLM
+  actions, including tool-use in subsequent conversation turns.  This
+  applies to context included via ~gptel-add~ or as a link in a
+  buffer.
+
 - Structured output support: ~gptel-request~ can now take an optional
   schema argument to constrain LLM output to the specified JSON
   schema.  The JSON schema can be provided as a serialized JSON string

gptel.el

@@ -996,7 +996,7 @@ Later plists in the sequence take precedence over earlier ones."
 
 (defun gptel--insert-file-string (path)
   "Insert at point the contents of the file at PATH as context."
-  (insert (format "In file `%s`:" (file-name-nondirectory path))
+  (insert (format "In file `%s`:" (abbreviate-file-name path))
           "\n\n```\n")
   (let ((pm (point-marker)))
     (set-marker-insertion-type pm t)