Merge branch 'js/misc-doc-fixes'
"make check-docs", "git help -a", etc. did not account for cases
where a particular build may deliberately omit some subcommands,
which has been corrected.
* js/misc-doc-fixes:
Turn `git serve` into a test helper
test-tool: handle the `-C <directory>` option just like `git`
check-docs: do not bother checking for legacy scripts' documentation
docs: exclude documentation for commands that have been excluded
check-docs: allow command-list.txt to contain excluded commands
help -a: do not list commands that are excluded from the build
Makefile: drop the NO_INSTALL variable
remote-testgit: move it into the support directory for t5801