[PATCH v4 0/6] tag: move PGP verification code to tag.c

This is a follow up of [1], [2], and [3]:

v4 (this):

Thanks Eric, Peff, and Hannes for the feedback.

 * I relocated the sigchain_push call so it comes after the error on
   gpg-interface (thanks Hannnes for catching this).
 * I updated the unit test to match the discussion on [3]. Now it generates
   the expected output of the tag on the fly for comparison. (This is just
   copy and paste from [3], but I verified that it works by breaking the
   while)
 * I split moving the code and renaming the variables into two patches so
   these are easier to review.
 * I used an adapter on builtin/tag.c instead of redefining all the fn*
   declarations everywhere. This introduces an issue with the way git tag -v
   resolves refnames though. I added a new commit to restore the previous
   behavior of git-tag. I'm not sure if I should've split this into two commits
   though.

v3: Thanks Eric, Jeff, for the feedback.

 * I separated the patch in multiple sub-patches.
 * I compared the behavior of previous git tag -v and git verify-tag 
   invocations to make sure the behavior is the same
 * I dropped the multi-line comment, as suggested.
 * I fixed the issue with the missing brackets in the while (this is 
   now detected by the test).

v2:

 * I moved the pgp-verification code to tag.c 
 * I added extra arguments so git tag -v and git verify-tag both work
   with the same function
 * Relocated the SIGPIPE handling code in verify-tag to gpg-interface

v1:
 
The verify tag function is just a thin wrapper around the verify-tag
command. We can avoid one fork call by doing the verification inside
the tag builtin instead.

This applies on v2.8.0.

Thanks! -Santiago

[1] http://git.661346.n2.nabble.com/PATCH-RFC-builtin-tag-c-move-PGP-verification-inside-builtin-td7651529.html#a7651547 [2] http://git.661346.n2.nabble.com/PATCH-tag-c-move-PGP-verification-code-from-plumbing-td7651562.html [3] http://git.661346.n2.nabble.com/PATCH-v3-0-4-tag-move-PGP-verification-code-to-tag-c-td7652334.html