Makefile: optionally symlink libexec/git-core binaries to bin/git

Add a INSTALL_SYMLINKS option which if enabled, changes the default
hardlink installation method to one where the relevant binaries in
libexec/git-core are symlinked back to ../../bin, instead of being
hardlinked.

This new option also overrides the behavior of the existing
NO_*_HARDLINKS variables which in some cases would produce symlinks
within to libexec/, e.g. "git-add" symlinked to "git" which would be
copy of the "git" found in bin/, now "git-add" in libexec/ is always
going to be symlinked to the "git" found in the bin/ directory.

This option is being added because:

 1) I think it makes what we're doing a lot more obvious. E.g. I'd
    never noticed that the libexec binaries were really just hardlinks
    since e.g. ls(1) won't show that in any obvious way. You need to
    start stat(1)-ing things and look at the inodes to see what's
    going on.

 2) Some tools have very crappy support for hardlinks, e.g. the Git
    shipped with GitLab is much bigger than it should be because
    they're using a chef module that doesn't know about hardlinks, see
    chef/omnibus#827

    I've also ran into other related issues that I think are explained
    by this, e.g. compiling git with debugging and rpm refusing to
    install a ~200MB git package with 2GB left on the FS, I think that
    was because it doesn't consider hardlinks, just the sum of the
    byte size of everything in the package.

As for the implementation, the "../../bin" noted above will vary given
some given some values of "../.." and "bin" depending on the depth of
the gitexecdir relative to the destdir, and the "bindir" target,
e.g. setting "bindir=/tmp/git/binaries gitexecdir=foo/bar/baz" will do
the right thing and produce this result:

    $ file /tmp/git/foo/bar/baz/git-add
    /tmp/git/foo/bar/baz/git-add: symbolic link to ../../../binaries/git

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>

Makefile

@@ -329,6 +329,13 @@ all::
 # when hardlinking a file to another name and unlinking the original file right
 # away (some NTFS drivers seem to zero the contents in that scenario).
 #
+# Define INSTALL_SYMLINKS if you prefer to have everything that can be
+# symlinked between bin/ and libexec/ to use relative symlinks between
+# the two. This option overrides NO_CROSS_DIRECTORY_HARDLINKS and
+# NO_INSTALL_HARDLINKS which will also use symlinking by indirection
+# within the same directory in some cases, INSTALL_SYMLINKS will
+# always symlink to the final target directly.
+#
 # Define NO_CROSS_DIRECTORY_HARDLINKS if you plan to distribute the installed
 # programs as a tar, where bin/ and libexec/ might be on different file systems.
 #
@@ -2594,35 +2601,44 @@ endif
 
 	bindir=$$(cd '$(DESTDIR_SQ)$(bindir_SQ)' && pwd) && \
 	execdir=$$(cd '$(DESTDIR_SQ)$(gitexec_instdir_SQ)' && pwd) && \
+	destdir_from_execdir_SQ=$$(echo '$(gitexecdir_relative_SQ)' | sed -e 's|[^/][^/]*|..|g') && \
 	{ test "$$bindir/" = "$$execdir/" || \
 	  for p in git$X $(filter $(install_bindir_programs),$(ALL_PROGRAMS)); do \
 		$(RM) "$$execdir/$$p" && \
-		test -z "$(NO_INSTALL_HARDLINKS)$(NO_CROSS_DIRECTORY_HARDLINKS)" && \
-		ln "$$bindir/$$p" "$$execdir/$$p" 2>/dev/null || \
-		cp "$$bindir/$$p" "$$execdir/$$p" || exit; \
+		test -n "$(INSTALL_SYMLINKS)" && \
+		ln -s "$$destdir_from_execdir_SQ/$(bindir_relative_SQ)/$$p" "$$execdir/$$p" || \
+		{ test -z "$(NO_INSTALL_HARDLINKS)$(NO_CROSS_DIRECTORY_HARDLINKS)" && \
+		  ln "$$bindir/$$p" "$$execdir/$$p" 2>/dev/null || \
+		  cp "$$bindir/$$p" "$$execdir/$$p" || exit; } \
 	  done; \
 	} && \
 	for p in $(filter $(install_bindir_programs),$(BUILT_INS)); do \
 		$(RM) "$$bindir/$$p" && \
-		test -z "$(NO_INSTALL_HARDLINKS)" && \
-		ln "$$bindir/git$X" "$$bindir/$$p" 2>/dev/null || \
-		ln -s "git$X" "$$bindir/$$p" 2>/dev/null || \
-		cp "$$bindir/git$X" "$$bindir/$$p" || exit; \
+		test -n "$(INSTALL_SYMLINKS)" && \
+		ln -s "git$X" "$$bindir/$$p" || \
+		{ test -z "$(NO_INSTALL_HARDLINKS)" && \
+		  ln "$$bindir/git$X" "$$bindir/$$p" 2>/dev/null || \
+		  ln -s "git$X" "$$bindir/$$p" 2>/dev/null || \
+		  cp "$$bindir/git$X" "$$bindir/$$p" || exit; } \
 	done && \
 	for p in $(BUILT_INS); do \
 		$(RM) "$$execdir/$$p" && \
-		test -z "$(NO_INSTALL_HARDLINKS)" && \
-		ln "$$execdir/git$X" "$$execdir/$$p" 2>/dev/null || \
-		ln -s "git$X" "$$execdir/$$p" 2>/dev/null || \
-		cp "$$execdir/git$X" "$$execdir/$$p" || exit; \
+		test -n "$(INSTALL_SYMLINKS)" && \
+		ln -s "$$destdir_from_execdir_SQ/$(bindir_relative_SQ)/git$X" "$$execdir/$$p" || \
+		{ test -z "$(NO_INSTALL_HARDLINKS)" && \
+		  ln "$$execdir/git$X" "$$execdir/$$p" 2>/dev/null || \
+		  ln -s "git$X" "$$execdir/$$p" 2>/dev/null || \
+		  cp "$$execdir/git$X" "$$execdir/$$p" || exit; } \
 	done && \
 	remote_curl_aliases="$(REMOTE_CURL_ALIASES)" && \
 	for p in $$remote_curl_aliases; do \
 		$(RM) "$$execdir/$$p" && \
-		test -z "$(NO_INSTALL_HARDLINKS)" && \
-		ln "$$execdir/git-remote-http$X" "$$execdir/$$p" 2>/dev/null || \
-		ln -s "git-remote-http$X" "$$execdir/$$p" 2>/dev/null || \
-		cp "$$execdir/git-remote-http$X" "$$execdir/$$p" || exit; \
+		test -n "$(INSTALL_SYMLINKS)" && \
+		ln -s "git-remote-http$X" "$$execdir/$$p" || \
+		{ test -z "$(NO_INSTALL_HARDLINKS)" && \
+		  ln "$$execdir/git-remote-http$X" "$$execdir/$$p" 2>/dev/null || \
+		  ln -s "git-remote-http$X" "$$execdir/$$p" 2>/dev/null || \
+		  cp "$$execdir/git-remote-http$X" "$$execdir/$$p" || exit; } \
 	done && \
 	./check_bindir "z$$bindir" "z$$execdir" "$$bindir/git-add$X"