Add list of commits for git blame to ignore (#1391)

This adds a list of commits which one may want to ignore when doing git blame
(git blame file.c --ignore-revs-file .git-blame-ignore-revs).

Formatting changes of Python (with Black), the initial C formatting commit,
and couple of other commits are included.
Includes Black formatting changes up to 21.5b1 update.

The intention is an ignore list for investigating 'standard' code. It won't work when
an error was introduced by the formatting change (e.g., by fixing some additional lines
manually) or when dealing with CI checks changed in the same commit.

Includes basic documentation of what the file is for.

.git-blame-ignore-revs

@@ -0,0 +1,25 @@
+# Format Python code with Black
+1aab3bbcff76c12d422e7dffd9aba8277f91cd86
+0d9e966dd289753cca91886529e65711eaf863eb
+57cb2d4bbe6ce71a8b97fce42dd4da736c18e636
+6d4d14afb3845a8ecdcef391f3375d97af4a6a84
+7e03dcd6a8d7d96ead53d0f72e57735050dc6c7a
+feff8e340f5fec1b52ce066360904f72501e76ce
+fd86f50e9a4c9ab37cf501290349790377846b59
+ff2c2290d1469972504753e5d1415eba5568e196
+e9dfbc4eee6fa14af2f0a8a5e1a7c723b34254db
+02bfae03d5db8c80a93a0e340ff3cdcd509a2785
+0111b11e8b3aae2c55f20eb1ceea3dc4ee86cee9
+# Major C formatting in 2008
+8868d4b6868e05345c93032704b7304de04c1daf
+# Per-file or per-directory C code formatting
+c08f63b1b604acbba275c144b543c5164bc9f432
+8eaef1f4e6e9497f469cbb7ab91fe03389a6b49b
+f6a73088a9c17b6cce7102c5c3107ab4b41f3b2c
+# parser: Use #-space-% to allow writing PEP8-compliant Python code (#1287)
+635224115c5ee9c29a9356210c3b03e5511ef6dc
+# Format shell/Bash scripts
+80bcf7a290a6a1249604b2c8a236154f13e25bfa
+a0ccdb551e004a0f68192fb65cfc9be61b2880ed
+# Format YAML in GH workflows (#1367)
+567fa0643d0ab48de590ffbf0e286a288d359312

doc/git/investigating_history.md

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+# Investigating Code History
+
+## Plain git blame
+
+In command line, to investigate commits which introduced a certain
+piece of code, you can use `git blame`:
+
+```sh
+git blame scripts/r.mask/r.mask.py
+```
+
+This will give you current content of the file with commit hash and author
+associated with each line.
+
+## Ignore some commits with git blame
+
+Over the course of time, there was a lot of code reformatting
+changes which you typically want to ignore. These, often whitespace-only
+changes, can be filtered out using a _ignore-revs_ file in the source code
+root directory:
+
+```sh
+git blame scripts/r.mask/r.mask.py --ignore-revs-file .git-blame-ignore-revs
+```
+
+In some cases, ignoring formatting changes may not be appropriate, for example
+when investigating automated checks of formatting.
+If you think the _ignore-revs_ file contains commits which should be never
+ignored with `git blame`, please open an issue.
+
+## On GitHub
+
+GitHub interface does not allow to use the ignore-revs file. However, it is
+easy to move over a revision by pressing "View blame prior to this change"
+which is the icon between commit info and line numbers.