[PATCH 1/1] Makefile: use `git ls-files` to list header files, if possible

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

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.

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>
---
 Makefile | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index c5240942f2..2644324915 100644
--- a/Makefile
+++ b/Makefile
@@ -841,7 +841,8 @@ VCSSVN_LIB = vcs-svn/lib.a
 
 GENERATED_H += command-list.h
 
-LIB_H = $(shell $(FIND) . \
+LIB_H = $(shell git ls-files '*.h' ':!t/' ':!Documentation/' 2>/dev/null || \
+	$(FIND) . \
 	-name .git -prune -o \
 	-name t -prune -o \
 	-name Documentation -prune -o \
@@ -2363,7 +2364,7 @@ else
 # should _not_ be included here, since they are necessary even when
 # building an object for the first time.
 
-$(OBJECTS): $(LIB_H)
+$(OBJECTS): $(LIB_H) $(GENERATED_H)
 endif
 
 exec-cmd.sp exec-cmd.s exec-cmd.o: GIT-PREFIX
-- 
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