[PATCH 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.

Let's just report that 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.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 Makefile  | 4 +++-
 help.c    | 2 ++
 version.c | 1 +
 version.h | 1 +
 4 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index fef9c8d2725..92a0ae3d8e3 100644
--- a/Makefile
+++ b/Makefile
@@ -1893,7 +1893,9 @@ 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 rev-parse -q --verify HEAD || \
+		echo "(unknown)")"'
 
 $(BUILT_INS): git$X
 	$(QUIET_BUILT_IN)$(RM) $@ && \
diff --git a/help.c b/help.c
index df1332fa3c9..6ebea665780 100644
--- a/help.c
+++ b/help.c
@@ -413,6 +413,8 @@ int cmd_version(int argc, const char **argv, const char *prefix)
 	printf("git version %s\n", git_version_string);
 
 	if (build_options) {
+		printf("built from commit: %s\n",
+		       git_built_from_commit_string);
 		printf("sizeof-long: %d\n", (int)sizeof(long));
 		printf("machine: %s\n", build_platform);
 		/* 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