Merge branch 'kf/askpass-config'

* kf/askpass-config:
Extend documentation of core.askpass and GIT_ASKPASS.
Allow core.askpass to override SSH_ASKPASS.
Add a new option 'core.askpass'.

diff --cc Documentation/config.txt
Simple merge

diff --cc Documentation/git.txt
Simple merge

diff --cc cache.h
Simple merge

diff --cc config.c
Simple merge

diff --cc environment.c
Simple merge

diff --cc git.c
Simple merge