debuginstall: expand the editor path before searching for it (issue4380)

The editor launches without expanding the path with commits because the shell
does that for us.

If the path isn't an executable, the expanded path is displayed, which is
probably more useful than the unexpanded path.  For example, in cmd.exe, '~'
expands to C:\Users\$user.  But it expands to C:/mingw/msys/1.0/home/$user in
MinGW.

mercurial/commands.py

@@ -2370,6 +2370,7 def debuginstall(ui):
     # editor
     ui.status(_("checking commit editor...\n"))
     editor = ui.geteditor()
+    editor = util.expandpath(editor)
     cmdpath = util.findexe(shlex.split(editor)[0])
     if not cmdpath:
         if editor == 'vi':

tests/test-install.t

@@ -24,3 +24,20 hg debuginstall with no username
    (specify a username in your configuration file)
   1 problems detected, please check your install!
   [1]
+
+path variables are expanded (~ is the same as $TESTTMP)
+  $ mkdir tools
+  $ touch tools/testeditor.exe
+#if execbit
+  $ chmod 755 tools/testeditor.exe
+#endif
+  $ hg debuginstall --config ui.editor=~/tools/testeditor.exe
+  checking encoding (ascii)...
+  checking Python executable (*) (glob)
+  checking Python version (*) (glob)
+  checking Python lib (*lib*)... (glob)
+  checking installed modules (*mercurial)... (glob)
+  checking templates (*mercurial?templates)... (glob)
+  checking commit editor...
+  checking username...
+  no problems detected