Merge branch 'jk/pretty-G-format-fixes' into maint

"%G" (nothing after G) is an invalid pretty format specifier, but
the parser did not notice it as garbage.

* jk/pretty-G-format-fixes:
move "%G" format test from t7510 to t6006
pretty: avoid reading past end-of-string with "%G"
t7510: check %G* pretty-format output
t7510: test a commit signed by an unknown key
t7510: use consistent &&-chains in loop
t7510: stop referring to master in later tests

diff --cc pretty.c
Simple merge

diff --cc t/t6006-rev-list-format.sh
Simple merge