Merge branch 'sp/runtime-prefix'

* sp/runtime-prefix:
Windows: Revert to default paths and convert them by RUNTIME_PREFIX
Compute prefix at runtime if RUNTIME_PREFIX is set
Modify setup_path() to only add git_exec_path() to PATH
Add calls to git_extract_argv0_path() in programs that call git_config_*
git_extract_argv0_path(): Move check for valid argv0 from caller to callee
Refactor git_set_argv0_path() to git_extract_argv0_path()
Move computation of absolute paths from Makefile to runtime (in preparation for RUNTIME_PREFIX)

diff --cc Makefile
Simple merge

diff --cc http-push.c
Simple merge

diff --cc index-pack.c
Simple merge