Make bob builds show the full source git commit hash in buildinfo.

The Windows binaries, and both Windows and Unix source archives,
output from a bob build will now include the full SHA-1 of the source
git commit in their buildinfo (hence in all the About boxes and
command-line version output).

This will be occasionally useful to me at release time (there was that
one embarrassing incident where I managed not to notice that I'd made
a release build from entirely the wrong commit), but mostly, it just
seems like an obviously useful thing to put in a general buildinfo
section now that there is one.

misc.c

@@ -1151,6 +1151,7 @@ int strendswith(const char *s, const char *t)
 char *buildinfo(const char *newline)
 {
     strbuf *buf = strbuf_new();
+    extern const char commitid[];      /* in commitid.c */
 
     strbuf_catf(buf, "Build platform: %d-bit %s",
                 (int)(CHAR_BIT * sizeof(void *)),
@@ -1207,5 +1208,7 @@ char *buildinfo(const char *newline)
     strbuf_catf(buf, "%sBuild option: DEBUG", newline);
 #endif
 
+    strbuf_catf(buf, "%sSource commit: %s", newline, commitid);
+
     return strbuf_to_str(buf);
 }