[PATCH v2 2/2] version --build-options: report commit, too, if possible

[Johannes Schindelin <johannes.schindelin@gmx.de>]

In particular when local tags are used (or tags that are pushed to some
fork) to build Git, it is very hard to figure out from which particular
revision a particular Git executable was built. It gets worse when those
tags are deleted, or even updated.

Let's just report an exact, unabbreviated commit name in our build
options.

We need to be careful, though, to report when the current commit cannot
be determined, e.g. when building from a tarball without any associated
Git repository. This could be the case also when extracting Git's source
code into an unrelated Git worktree.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 Makefile  | 4 +++-
 help.c    | 5 +++++
 version.c | 1 +
 version.h | 1 +
 4 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 5587bccc932..2ce70d205d9 100644
--- a/Makefile
+++ b/Makefile
@@ -1902,7 +1902,9 @@ builtin/help.sp builtin/help.s builtin/help.o: EXTRA_CPPFLAGS = \
 version.sp version.s version.o: GIT-VERSION-FILE GIT-USER-AGENT
 version.sp version.s version.o: EXTRA_CPPFLAGS = \
 	'-DGIT_VERSION="$(GIT_VERSION)"' \
-	'-DGIT_USER_AGENT=$(GIT_USER_AGENT_CQ_SQ)'
+	'-DGIT_USER_AGENT=$(GIT_USER_AGENT_CQ_SQ)' \
+	'-DGIT_BUILT_FROM_COMMIT="$(shell GIT_CEILING_DIRECTORIES=\"$(CURDIR)/..\" \
+		git rev-parse -q --verify HEAD || :)"'
 
 $(BUILT_INS): git$X
 	$(QUIET_BUILT_IN)$(RM) $@ && \
diff --git a/help.c b/help.c
index cbcb159f367..60071a9beaa 100644
--- a/help.c
+++ b/help.c
@@ -413,6 +413,11 @@ int cmd_version(int argc, const char **argv, const char *prefix)
 
 	if (build_options) {
 		printf("cpu: %s\n", GIT_HOST_CPU);
+		if (git_built_from_commit_string[0])
+			printf("built from commit: %s\n",
+			       git_built_from_commit_string);
+		else
+			printf("no commit associated with this build\n");
 		printf("sizeof-long: %d\n", (int)sizeof(long));
 		/* NEEDSWORK: also save and output GIT-BUILD_OPTIONS? */
 	}
diff --git a/version.c b/version.c
index 6106a8098c8..41b718c29e1 100644
--- a/version.c
+++ b/version.c
@@ -3,6 +3,7 @@
 #include "strbuf.h"
 
 const char git_version_string[] = GIT_VERSION;
+const char git_built_from_commit_string[] = GIT_BUILT_FROM_COMMIT;
 
 const char *git_user_agent(void)
 {
diff --git a/version.h b/version.h
index 6911a4f1558..7c62e805771 100644
--- a/version.h
+++ b/version.h
@@ -2,6 +2,7 @@
 #define VERSION_H
 
 extern const char git_version_string[];
+extern const char git_built_from_commit_string[];
 
 const char *git_user_agent(void);
 const char *git_user_agent_sanitized(void);
-- 
2.15.1.windows.2