Merge branch 'cc/replace'

* cc/replace:
t6050: check pushing something based on a replaced commit
Documentation: add documentation for "git replace"
Add git-replace to .gitignore
builtin-replace: use "usage_msg_opt" to give better error messages
parse-options: add new function "usage_msg_opt"
builtin-replace: teach "git replace" to actually replace
Add new "git replace" command
environment: add global variable to disable replacement
mktag: call "check_sha1_signature" with the replacement sha1
replace_object: add a test case
object: call "check_sha1_signature" with the replacement sha1
sha1_file: add a "read_sha1_file_repl" function
replace_object: add mechanism to replace objects found in "refs/replace/"
refs: add a "for_each_replace_ref" function

diff --cc Makefile
Simple merge

diff --cc builtin-fsck.c
Simple merge

diff --cc builtin-pack-objects.c
Simple merge

diff --cc builtin-unpack-objects.c
Simple merge

diff --cc builtin.h
Simple merge

diff --cc cache.h
Simple merge

diff --cc commit.h
Simple merge

diff --cc environment.c
Simple merge

diff --cc git.c
Simple merge

diff --cc mktag.c
Simple merge

diff --cc parse-options.c
Simple merge

diff --cc parse-options.h
Simple merge

diff --cc refs.c
Simple merge

diff --cc sha1_file.c
Simple merge

diff --cc upload-pack.c
Simple merge