Merge branch 'jc/diff-unique-abbrev-comments' into maint

A bit more comments in a tricky code.

* jc/diff-unique-abbrev-comments:
  diff_unique_abbrev(): document its assumption and limitation

diff.c

@@ -4089,7 +4089,8 @@ void diff_free_filepair(struct diff_filepair *p)
 	free(p);
 }
 
-/* This is different from find_unique_abbrev() in that
+/*
+ * This is different from find_unique_abbrev() in that
  * it stuffs the result with dots for alignment.
  */
 const char *diff_unique_abbrev(const unsigned char *sha1, int len)
@@ -4101,6 +4102,26 @@ const char *diff_unique_abbrev(const unsigned char *sha1, int len)
 
 	abbrev = find_unique_abbrev(sha1, len);
 	abblen = strlen(abbrev);
+
+	/*
+	 * In well-behaved cases, where the abbbreviated result is the
+	 * same as the requested length, append three dots after the
+	 * abbreviation (hence the whole logic is limited to the case
+	 * where abblen < 37); when the actual abbreviated result is a
+	 * bit longer than the requested length, we reduce the number
+	 * of dots so that they match the well-behaved ones.  However,
+	 * if the actual abbreviation is longer than the requested
+	 * length by more than three, we give up on aligning, and add
+	 * three dots anyway, to indicate that the output is not the
+	 * full object name.  Yes, this may be suboptimal, but this
+	 * appears only in "diff --raw --abbrev" output and it is not
+	 * worth the effort to change it now.  Note that this would
+	 * likely to work fine when the automatic sizing of default
+	 * abbreviation length is used--we would be fed -1 in "len" in
+	 * that case, and will end up always appending three-dots, but
+	 * the automatic sizing is supposed to give abblen that ensures
+	 * uniqueness across all objects (statistically speaking).
+	 */
 	if (abblen < 37) {
 		static char hex[41];
 		if (len < abblen && abblen <= len + 2)