examinekey: Added --migrate-to-backend

Note that, the way the SeekInput parser is written to support batch mode,
it's actually possible to do git-annex examinekey
"SHA1--foo foo.tar.gz" --migrate-to-backend=SHA1E

While that might be kind of useful to support multiple migrations not using
batch mode, I have not documented it. It would be better to take pairs of
key and file in that case.

doc/git-annex-examinekey.mdwn

@@ -42,11 +42,37 @@ that can be determined purely by looking at the key.
   Messages that would normally be output to standard error are included in
   the json instead.
 
+* `--migrate-to-backend=backend`
+
+  Attempt to migrate the input key to the new backend specified. If
+  successful, outputs information about the migrated key. Otherwise,
+  outputs information about the input key.
+
+  This only does fast migrations; it will not re-hash the content of a key
+  or similar expensive operation.
+
+  One way to use it is to add an extension to a key.
+
+	git-annex examinekey SHA256--xxx --migrate-to-backend=SHA256E --filename=foo.tar.gz
+
+  Or to remove the extension from a key:
+
+  	git-annex examinekey SHA256E-xxx.tar.gz --migrate-to-backend=SHA256
+
+* `--filename=name`
+
+  The name of a file associated with the key, eg a work tree file.
+  It does not need to exist. This is needed when using `--migrate-to-backend`
+  to add an extension to the key.
+
 * `--batch`
 
   Enable batch mode, in which a line containing a key is read from stdin,
   the information about it is output to stdout, and repeat.
 
+  In order to also provide the name of a file associated with the key, the
+  line can be in the format "$key $file"
+
 # SEE ALSO
 
 [[git-annex]](1)

doc/todo/calckey___91__--path__93___given_a_checksum_and_filename__63__.mdwn

@@ -8,3 +8,5 @@ edit1: I was pointed to [forum/Is_it_possible_adding_files_to_git_annex_bypassin
 
 [[!meta author=yoh]]
 [[!tag projects/datalad]]
+
+> [[done]] --[[Joey]]

doc/todo/calckey___91__--path__93___given_a_checksum_and_filename__63__/comment_4_ee750e205acefe400d13c3a5e967ca3b._comment

@@ -0,0 +1,7 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 4"""
+ date="2020-11-12T18:08:37Z"
+ content="""
+Implemented examinekey --migrate-to-backend
+"""]]