pm_apply: improve regexp

[nephros]

corrects regular expressions:

1. '+' has special meaning, escape it
2. handle patches with "git-style" prefixes in the diff header, see Test Cases #232 (comment)

[Olf0]

Notes:

1. Ad "1. '+' has special meaning, escape it":
   AFAIK only for PCREs (Pearl complex REs) and maybe for the obsolete EREs, but sed is special so often and I regularly become confused between the various RE-styles: It can do no harm to escape each +.
2. Ad "2. handle patches with "git-style" prefixes in the diff header":
   AFAIU from the diff documentation, these are also simply path elements, but of relative paths. See GNU Diffutils: Detailed Description of Unified Format and especially GNU Diffutils: Avoiding Common Mistakes.
   I intended to argue that the character set for the first path element should be much more limited than [^/] (e.g., to [[:alnum:]]), but then realised that it could even contain backslash-quoted spaces ("\ "). So I have no better idea how to handle this.

[Olf0]

Looking O.K.

[nephros]

I intended to argue that the character set for the first path element should be much more limited than [^/] (e.g., to [[:alnum:]]), but then realised that it could even contain backslash-quoted spaces ("\ "). So I have no better idea how to handle this.

Yeah I was reluctant to use such broad matching. Correct one would be "any character allowed in a linux file path, including illegal characters if escaped properly". But even if such a regex term exists it would probably just look like line noise, hence not very maintainable.
So I opted for the easily readable might-break-in-some-corner-cases option.