When trying to use C-x v v in a vc-diff buffer, and when the version
control system is Fossil (this is using GNU patch):
Checking in foo...
user-error: Patch failed: can't find file to patch at input line 5
Perhaps you used the wrong -p or --strip option?
The text leading up to this was:
--------------------------
|Index: foo
|==================================================================
|--- foo
|+++ foo
--------------------------
File to patch:
Skip this patch? [y]
Skipping patch.
1 out of 1 hunk ignored
To reproduce, make a empty Fossil repostiroy, add and commit a file,
add a line, C-x v =, C-x v v. vc-fossil has to be installed.