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
syntax: glob
*.elc
*.tmp
*.orig
*.rej
*~
*.mergebackup
*.o
*.so
*.dll
*.exe
*.pyd
*.pyc
*.pyo
*$py.class
*.swp
*.prof
*.zip
\#*\#
.\#*
tests/artifacts/cache/big-file-churn.hg
tests/.coverage*
tests/.testtimes*
tests/.hypothesis
tests/hypothesis-generated
tests/annotated
tests/exceptions
tests/*.err
tests/htmlcov
build
contrib/chg/chg
contrib/hgsh/hgsh
contrib/vagrant/.vagrant
dist
packages
doc/common.txt
doc/*.[0-9]
doc/*.[0-9].txt
doc/*.[0-9].gendoc.txt
doc/*.[0-9].{x,ht}ml
MANIFEST
MANIFEST.in
patches
mercurial/__modulepolicy__.py
mercurial/__version__.py
mercurial/hgpythonlib.h
mercurial.egg-info
.DS_Store
tags
cscope.*
.vscode/*
.idea/*
.asv/*
.pytype/*
.mypy_cache
i18n/hg.pot
locale/*/LC_MESSAGES/hg.mo
hgext/__index__.py
rust/target/
rust/*/target/
# Generated wheels
wheelhouse/
syntax: rootglob
# See Profiling in rust/README.rst
.cargo/config
syntax: regexp
^\.pc/
^\.(pydev)?project
# hackable windows distribution additions
^hg-python
^hg.py$