show basename in source: split line

before this change I would get a split with...
`────────────────────────── source:/server/externa[...].cpp+130 ────`
which does not tell me anything about what file I am in.

now I get something like this...
`─────────────── source:/server/externa[...]SendReceive.hpp+110 ────`

gef.py

@@ -7663,8 +7663,11 @@ def context_source(self) -> None:
 
         nb_line = self["nb_lines_code"]
         fn = symtab.filename
-        if len(fn) > 20:
-            fn = f"{fn[:15]}[...]{os.path.splitext(fn)[1]}"
+        if len(fn) > 30:
+            base = os.path.basename(fn)
+            if len(base) > 20:
+                base = f"{base[-10:]}"
+            fn = f"{fn[:15]}[...]{base}"
         title = f"source:{fn}+{line_num + 1}"
         cur_line_color = gef.config["theme.source_current_line"]
         self.context_title(title)