Document git_buf_common_prefix

This function fills in a git_buf with the common prefix of
an array of strings, but let's make that a little more clear.

src/buffer.c

@@ -426,10 +426,13 @@ int git_buf_common_prefix(git_buf *buf, const git_strarray *strings)
 	if (!strings || !strings->count)
 		return 0;
 
+	/* initialize common prefix to first string */
 	if (git_buf_sets(buf, strings->strings[0]) < 0)
 		return -1;
 
+	/* go through the rest of the strings, truncating to shared prefix */
 	for (i = 1; i < strings->count; ++i) {
+
 		for (str = strings->strings[i], pfx = buf->ptr;
 			 *str && *str == *pfx; str++, pfx++)
 			/* scanning */;

src/buffer.h

@@ -122,6 +122,7 @@ void git_buf_rtrim(git_buf *buf);
 
 int git_buf_cmp(const git_buf *a, const git_buf *b);
 
+/* Fill buf with the common prefix of a array of strings */
 int git_buf_common_prefix(git_buf *buf, const git_strarray *strings);
 
 #endif