shelve: rewrite check for unknown shelf to delete

shelve: rewrite check for unknown shelf to delete

The code would try to delete the shelf's .patch file and if that
raised an exception, it would convert it to an `error.Abort`. This
patch rewrites it so the check is done upfront. I find it easier to
read that way. It's now clear enough that I removed the comment
explaining it as well.

As Joerg pointed out during review, another differences is that the
old code would move a `.hg` file without its `.patch` friend to backup
before it realized that the `.patch` file was missing. The new code
will error out earlier and not move the `.hg` file, which seems like
an improvement. That should only matter on corrupt `.hg/shelved/`
directories, however.

Differential Revision: https://phab.mercurial-scm.org/D9697

--- a/mercurial/shelve.py	Thu Jan 07 12:37:14 2021 -0800
+++ b/mercurial/shelve.py	Thu Jan 07 12:22:39 2021 -0800
@@ -600,20 +600,12 @@
         raise error.Abort(_(b'no shelved changes specified!'))
     with repo.wlock():
         for name in pats:
-            try:
-                for suffix in shelvefileextensions:
-                    shfile = shelvedfile(repo, name, suffix)
-                    # patch file is necessary, as it should
-                    # be present for any kind of shelve,
-                    # but the .hg file is optional as in future we
-                    # will add obsolete shelve with does not create a
-                    # bundle
-                    if shfile.exists() or suffix == patchextension:
-                        shfile.movetobackup()
-            except OSError as err:
-                if err.errno != errno.ENOENT:
-                    raise
+            if not shelvedfile(repo, name, patchextension).exists():
                 raise error.Abort(_(b"shelved change '%s' not found") % name)
+            for suffix in shelvefileextensions:
+                shfile = shelvedfile(repo, name, suffix)
+                if shfile.exists():
+                    shfile.movetobackup()
             cleanupoldbackups(repo)
 
 

--- a/tests/test-shelve2.t	Thu Jan 07 12:37:14 2021 -0800
+++ b/tests/test-shelve2.t	Thu Jan 07 12:22:39 2021 -0800
@@ -776,8 +776,8 @@
   $ find .hg/shelve* | sort
   .hg/shelve-backup
   .hg/shelve-backup/junk1.patch
-  .hg/shelve-backup/junk2.hg
   .hg/shelved
+  .hg/shelved/junk2.hg
 
 # A file with an unexpected extension
   $ touch .hg/shelved/junk3
@@ -791,8 +791,8 @@
   $ find .hg/shelve* | sort
   .hg/shelve-backup
   .hg/shelve-backup/junk1.patch
-  .hg/shelve-backup/junk2.hg
   .hg/shelved
+  .hg/shelved/junk2.hg
   .hg/shelved/junk3
 
   $ cd ..