extdiff: refactor cmdline and gui calculation login in a separate fn

This is some good logic with handling of many config items and various ways of
specifying it. I want to reuse in `diff --tool` and hence refatcored it in a
separate function of it's own.

hgext/extdiff.py

@@ -709,10 +709,24 b' class savedcmd(object):'
         )
 
 
-def uisetup(ui):
-    for cmd, path in ui.configitems(b'extdiff'):
-        if cmd.startswith(b'opts.') or cmd.startswith(b'gui.'):
-            continue
+def _gettooldetails(ui, cmd, path):
+    """
+    returns following things for a
+    ```
+    [extdiff]
+    <cmd> = <path>
+    ```
+    entry:
+
+    cmd: command/tool name
+    path: path to the tool
+    cmdline: the command which should be run
+    isgui: whether the tool uses GUI or not
+
+    Reads all external tools related configs, whether it be extdiff section,
+    diff-tools or merge-tools section, or its specified in an old format or
+    the latest format.
+    """
         path = util.expandpath(path)
         if cmd.startswith(b'cmd.'):
             cmd = cmd[4:]
@@ -748,6 +762,14 b' def uisetup(ui):'
                     if isgui is None:
                         isgui = ui.configbool(section, cmd + b'.gui') or False
                     break
+    return cmd, path, cmdline, isgui
+
+
+def uisetup(ui):
+    for cmd, path in ui.configitems(b'extdiff'):
+        if cmd.startswith(b'opts.') or cmd.startswith(b'gui.'):
+            continue
+        cmd, path, cmdline, isgui = _gettooldetails(ui, cmd, path)
         command(
             cmd,
             extdiffopts[:],