Merge branch 'jc/am-3-nonstandard-popt' into maint

The code to synthesize the fake ancestor tree used by 3-way merge
fallback in "git am" was not prepared to read a patch created with
a non-standard -p<num> value.

* jc/am-3-nonstandard-popt:
test: "am -3" can accept non-standard -p<num>
am -3: allow nonstandard -p<num> option

diff --cc git-am.sh
Simple merge

diff --cc t/t4150-am.sh
Simple merge