Makefile: use `git ls-files` to list header files, if possible

In d85b0dff72 (Makefile: use `find` to determine static header
dependencies, 2014-08-25), we switched from a static list of header
files to a dynamically-generated one, asking `find` to enumerate them.

Back in those days, we did not use `$(LIB_H)` by default, and many a
`make` implementation seems smart enough not to run that `find` command
in that case, so it was deemed okay to run `find` for special targets
requiring this macro.

However, as of ebb7baf02f (Makefile: add a hdr-check target,
2018-09-19), $(LIB_H) is part of a global rule and therefore must be
expanded. Meaning: this `find` command has to be run upon every
`make` invocation. In the presence of many a worktree, this can tax the
developers' patience quite a bit.

Even in the absence of worktrees or other untracked files and
directories, the cost of I/O to generate that list of header files is
simply a lot larger than a simple `git ls-files` call.

Therefore, just like in 335339758c (Makefile: ask "ls-files" to list
source files if available, 2011-10-18), we now prefer to use `git
ls-files` to enumerate the header files to enumerating them via `find`,
falling back to the latter if the former failed (which would be the case
e.g. in a worktree that was extracted from a source .tar file rather
than from a clone of Git's sources).

This has one notable consequence: we no longer include `command-list.h`
in `LIB_H`, as it is a generated file, not a tracked one, but that is
easily worked around. Of the three sites that use `LIB_H`, two
(`LOCALIZED_C` and `CHK_HDRS`) already handle generated headers
separately. In the third, the computed-dependency fallback, we can just
add in a reference to $(GENERATED_H).

Likewise, we no longer include not-yet-tracked header files in `LIB_H`.

Given the speed improvements, these consequences seem a comparably small
price to pay.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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