Expose gpgsig in pretty-print

Currently, users who do not have GPG installed have no way to discern signed from unsigned commits without examining raw commit data. I propose two new pretty-print placeholders to expose this information:

%GR: full ("R"aw) contents of gpgsig header %G+: Y/N if the commit has nonempty gpgsig header or not

The second is of course much more likely to be used, but having exposed the one, exposing the other too adds almost no complexity.

I'm open to suggestion on the names of these placeholders.

This commit is based on master but e5a329a279 ("run-command: report exec failure" 2018-12-11) is required for the tests to pass.

One note is that this change touches areas of the pretty-format documentation that are radically revamped in aw/pretty-trailers: see 42617752d4 ("doc: group pretty-format.txt placeholders descriptions" 2018-12-08). I have another version of this branch based on that branch as well, so you can use that in case conflicts with aw/pretty-trailers arise.

See:
- https://github.com/jpassaro/git/tree/jp/pretty-expose-gpgsig
- https://github.com/jpassaro/git/tree/jp/pretty-expose-gpgsig--based-on-aw-pretty-trailers

John Passaro (4):
pretty: expose raw commit signature t/t7510-signed-commit.sh: test new placeholders doc, tests: pretty behavior when gpg missing docs/pretty-formats: add explanation + copy edits

 Documentation/pretty-formats.txt |  21 ++++--
 pretty.c                         |  36 ++++++++-
 t/t7510-signed-commit.sh         | 125 +++++++++++++++++++++++++++++--
 3 files changed, 167 insertions(+), 15 deletions(-)

base-commit: 5d826e972970a784bd7a7bdf587512510097b8c7
prerequisite-patch-id: aedfe228fd293714d9cd0392ac22ff1cba7365db

2.19.1