bug#61668: Bug in flymake-proc with fix

[Camden Narzt]

I’m sorry if this is not the correct place to report a flymake-proc bug, but 
since flymake is included in the emacs git repo I figured it might be ok.

I’m currently seeing incorrect behaviour from the 
`flymake-proc--delete-temp-directory` function. The path is parsed and then 
reassembled incorrectly as the following backtrace extract demonstrates:

flymake-proc--safe-delete-directory("/private/var/folders/p7/03_g5t611499lmjqhwc5tljr0000gn/T/000gn/T/Users/camdennarzt/Developer/Java/getargv.java/src/main/java/cam/narzt/getargv")
flymake-proc--delete-temp-directory("/private/var/folders/p7/03_g5t611499lmjqhwc5tljr0000gn/T/Users/camdennarzt/Developer/Java/getargv.java/src/main/java/cam/narzt/getargv/“)

As you can see the `000gn/T/` segment of the path gets duplicated when 
`flymake-proc--safe-delete-directory` gets called.

This is because in `flymake-proc--delete-temp-directory` when the `suffix` 
variable is declared it is assumed that `(directory-file-name 
temporary-file-directory)` is a prefix of the `dir-name` argument, however 
`(directory-file-name temporary-file-directory)` doesn’t seem to resolve 
symlinks in the path whereas `dir-name` seems to have symlinks already 
resolved, so they don’t necessarily match. On my system the difference is:

(directory-file-name temporary-file-directory) → 
"/var/folders/p7/03_g5t611499lmjqhwc5tljr0000gn/T”
dir-name → 
"/private/var/folders/p7/03_g5t611499lmjqhwc5tljr0000gn/T/Users/camdennarzt/Developer/Java/getargv.java/src/main/java/cam/narzt/getargv/“

Note the "/private” prefix on the `dir-name` path. That difference in length 
causes the suffix to be incorrectly determined and then causes a bunch of 
errors while any subsequent function call tries to work with a path with the 
`000gn/T/` segment duplicated which obviously doesn’t exist in the fs.

Changing the suffix variable to be computed as follows fixes the bug:

(substring dir-name (1+ (length (file-truename (expand-file-name 
(directory-file-name temp-dir))))))


If this can be fixed that’s great, if there’s somewhere else I should report 
this I’d love to know.

Cheers,

Camden Narzt