How does "index f2e4113..d4b9bfc 100644" in git diff correspond to SHA1 ID in gitk?

Question

edit: @wiswit has pointed to this good explanation.

f2e4113 and d4b9bfc are file-indexes. With git show f2e4113 you see the file before the commit and git show d4b9bfc shows the file after the commit.

The combined diff format is in the git diff manual pages described.

COMBINED DIFF FORMAT
   Any diff-generating command can take the ‘-c` or --cc option to produce a 
   combined diff when showing a merge. This is the default format when showing 
   merges with git-diff(1) or git-show(1). Note also that you can give the `-m’
   option to any of these commands to force generation of diffs with individual 
   parents of a merge.

   A combined diff format looks like this:

       diff --combined describe.c
       index fabadb8,cc95eb0..4866510
       --- a/describe.c
       +++ b/describe.c
       @@@ -98,20 -98,12 +98,20 @@@
               return (a_date > b_date) ? -1 : (a_date == b_date) ? 0 : 1;
         }

       - static void describe(char *arg)
        -static void describe(struct commit *cmit, int last_one)
       ++static void describe(char *arg, int last_one)
         {
        +      unsigned char sha1[20];
        +      struct commit *cmit;
               struct commit_list *list;
               static int initialized = 0;
               struct commit_name *n;

        +      if (get_sha1(arg, sha1) < 0)
        +              usage(describe_usage);
        +      cmit = lookup_commit_reference(sha1);
        +      if (!cmit)
        +              usage(describe_usage);
        +
               if (!initialized) {
                       initialized = 1;
                       for_each_ref(get_name);

    1. It is preceded with a "git diff" header, that looks like this (when -c 
       option is used):

           diff --combined file

       or like this (when --cc option is used):

           diff --cc file

    2. It is followed by one or more extended header lines (this example shows
       a merge with two parents):

           index <hash>,<hash>..<hash>
           mode <mode>,<mode>..<mode>
           new file mode <mode>
           deleted file mode <mode>,<mode>

       The mode <mode>,<mode>..<mode> line appears only if at least one of the 
       <mode> is different from the rest. Extended headers with information 
       about  detected contents movement (renames and copying detection) are
       designed to work with diff of two <tree-ish> and are not used by combined
       diff format.

    3. It is followed by two-line from-file/to-file header

           --- a/file
           +++ b/file

       Similar to two-line header for traditional unified diff format, 
       /dev/null is used to signal created or deleted files.

    4. Chunk header format is modified to prevent people from accidentally
       feeding it to patch -p1. Combined diff format was created for review 
       of merge commit changes, and was not meant for apply. The change is 
       similar to the change in the extended index header:

           @@@ <from-file-range> <from-file-range> <to-file-range> @@@

       There are (number of parents + 1) @ characters in the chunk header for 
       combined diff format.

   Unlike the traditional unified diff format, which shows two files A and B
   with a single column that has - (minus — appears in A but removed in B),
   + (plus — missing in A but added to B), or " " (space — unchanged) prefix,
   this format compares two or more files file1, file2,... with one file X, 
   and shows how X differs from each of fileN. One column for each of fileN is
   prepended to the output line to note how X’s line is different from it.

   A - character in the column N means that the line appears in fileN but it
   does not appear in the result. A + character in the column N means that
   the line appears in the result, and fileN does not have that line (in 
   other words, the line was added, from the point of view of that parent).

   In the above example output, the function signature was changed from both
   files (hence two - removals from both file1 and file2, plus ++ to mean one 
   line that was added does not appear in either file1 nor file2). Also eight 
   other lines are the same from file1 but do not appear in file2 (hence 
   prefixed with +).

   When shown by git diff-tree -c, it compares the parents of a merge commit
   with the merge result (i.e. file1..fileN are the parents). When shown by 
   git diff-files -c, it compares the two unresolved merge parents with the 
   working tree file (i.e. file1 is stage 2 aka "our version", file2 is stage 
   3 aka "their version").